diff --git a/utest/CMakeLists.txt b/utest/CMakeLists.txt
index 687c75ab..b1662736 100644
--- a/utest/CMakeLists.txt
+++ b/utest/CMakeLists.txt
@@ -7,6 +7,7 @@ set(UTEST_SRCS
     X1Collector.test.cpp
     DConstant.test.cpp
     DVariable.test.cpp
+    DApplyExpr.test.cpp
 )
 
 xo_add_utest_executable(${UTEST_EXE} ${UTEST_SRCS})
diff --git a/utest/DApplyExpr.test.cpp b/utest/DApplyExpr.test.cpp
new file mode 100644
index 00000000..d151b776
--- /dev/null
+++ b/utest/DApplyExpr.test.cpp
@@ -0,0 +1,309 @@
+/** @file DApplyExpr.test.cpp
+ *
+ *  @author Roland Conybeare, Jan 2026
+ **/
+
+#include "init_expression2.hpp"
+#include <xo/expression2/DApplyExpr.hpp>
+#include <xo/expression2/DConstant.hpp>
+#include <xo/expression2/detail/IExpression_DApplyExpr.hpp>
+#include <xo/expression2/detail/IExpression_DConstant.hpp>
+#include <xo/expression2/detail/IPrintable_DApplyExpr.hpp>
+#include <xo/expression2/detail/IPrintable_DConstant.hpp>
+
+#include <xo/procedure2/init_primitives.hpp>
+#include <xo/procedure2/detail/IGCObject_DPrimitive_gco_2_gco_gco.hpp>
+#include <xo/procedure2/detail/IPrintable_DPrimitive_gco_2_gco_gco.hpp>
+
+#include <xo/object2/DFloat.hpp>
+#include <xo/object2/number/IGCObject_DFloat.hpp>
+
+#include <xo/gc/CollectorTypeRegistry.hpp>
+#include <xo/gc/DX1Collector.hpp>
+#include <xo/gc/detail/IAllocator_DX1Collector.hpp>
+#include <xo/gc/detail/ICollector_DX1Collector.hpp>
+
+#include <xo/printable2/Printable.hpp>
+#include <xo/facet/FacetRegistry.hpp>
+#include <xo/reflect/Reflect.hpp>
+
+#include <xo/indentlog/scope.hpp>
+
+#include <catch2/catch.hpp>
+
+#include <sstream>
+
+namespace ut {
+    using xo::S_expression2_tag;
+    using xo::scm::DApplyExpr;
+    using xo::scm::DConstant;
+    using xo::scm::DFloat;
+    using xo::scm::AExpression;
+    using xo::scm::TypeRef;
+    using xo::scm::Primitives;
+    using xo::scm::DPrimitive_gco_2_gco_gco;
+    using xo::mm::CollectorTypeRegistry;
+    using xo::mm::AAllocator;
+    using xo::mm::ACollector;
+    using xo::mm::AGCObject;
+    using xo::mm::DX1Collector;
+    using xo::mm::CollectorConfig;
+    using xo::mm::ArenaConfig;
+    using xo::print::APrintable;
+    using xo::print::ppstate_standalone;
+    using xo::print::ppconfig;
+    using xo::facet::FacetRegistry;
+    using xo::facet::with_facet;
+    using xo::facet::obj;
+    using xo::reflect::Reflect;
+    using xo::InitEvidence;
+    using xo::InitSubsys;
+    using xo::scope;
+
+    static InitEvidence s_init = InitSubsys<S_expression2_tag>::require();
+
+    TEST_CASE("DApplyExpr-init", "[expression2][DApplyExpr]")
+    {
+        REQUIRE(s_init.evidence());
+    }
+
+    TEST_CASE("DApplyExpr-make2", "[expression2][DApplyExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "dapplyexpr_make2_test",
+            .arena_config_ = ArenaConfig{
+                .size_ = 8192,
+                .store_header_flag_ = true},
+            .object_types_z_ = 16384,
+            .gc_trigger_v_{{4096, 4096}},
+            .debug_flag_ = false,
+        };
+
+        DX1Collector gc(cfg);
+        auto alloc = with_facet<AAllocator>::mkobj(&gc);
+        auto coll = with_facet<ACollector>::mkobj(&gc);
+
+        bool ok = CollectorTypeRegistry::instance().install_types(coll);
+        REQUIRE(ok);
+
+        // wrap primitive as GCObject, then as expression
+        obj<AGCObject> prim_gco = with_facet<AGCObject>::mkobj(&Primitives::s_mul_gco_gco_pm);
+        obj<AExpression,DConstant> fn_expr = DConstant::make(alloc, prim_gco);
+        REQUIRE(fn_expr.data() != nullptr);
+
+        // create argument expressions
+        obj<AGCObject> val1 = DFloat::box<AGCObject>(alloc, 3.0);
+        obj<AGCObject> val2 = DFloat::box<AGCObject>(alloc, 7.0);
+        obj<AExpression,DConstant> arg1 = DConstant::make(alloc, val1);
+        obj<AExpression,DConstant> arg2 = DConstant::make(alloc, val2);
+        REQUIRE(arg1.data() != nullptr);
+        REQUIRE(arg2.data() != nullptr);
+
+        // create apply expression: mul(3.0, 7.0)
+        TypeRef result_type = TypeRef::resolved(Reflect::require<double>());
+        auto apply_expr = DApplyExpr::make2(alloc, result_type, fn_expr, arg1, arg2);
+        REQUIRE(apply_expr.data() != nullptr);
+    }
+
+    TEST_CASE("DApplyExpr-extype", "[expression2][DApplyExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "dapplyexpr_extype_test",
+            .arena_config_ = ArenaConfig{
+                .size_ = 8192,
+                .store_header_flag_ = true},
+            .object_types_z_ = 16384,
+            .gc_trigger_v_{{4096, 4096}},
+            .debug_flag_ = false,
+        };
+
+        DX1Collector gc(cfg);
+        auto alloc = with_facet<AAllocator>::mkobj(&gc);
+        auto coll = with_facet<ACollector>::mkobj(&gc);
+
+        bool ok = CollectorTypeRegistry::instance().install_types(coll);
+        REQUIRE(ok);
+
+        obj<AGCObject> prim_gco = with_facet<AGCObject>::mkobj(&Primitives::s_mul_gco_gco_pm);
+        obj<AExpression,DConstant> fn_expr = DConstant::make(alloc, prim_gco);
+
+        obj<AGCObject> val1 = DFloat::box<AGCObject>(alloc, 3.0);
+        obj<AGCObject> val2 = DFloat::box<AGCObject>(alloc, 7.0);
+        obj<AExpression,DConstant> arg1 = DConstant::make(alloc, val1);
+        obj<AExpression,DConstant> arg2 = DConstant::make(alloc, val2);
+
+        TypeRef result_type = TypeRef::resolved(Reflect::require<double>());
+        auto apply_expr = DApplyExpr::make2(alloc, result_type, fn_expr, arg1, arg2);
+
+        REQUIRE(apply_expr.data()->extype() == xo::scm::exprtype::apply);
+    }
+
+    TEST_CASE("DApplyExpr-n_args", "[expression2][DApplyExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "dapplyexpr_n_args_test",
+            .arena_config_ = ArenaConfig{
+                .size_ = 8192,
+                .store_header_flag_ = true},
+            .object_types_z_ = 16384,
+            .gc_trigger_v_{{4096, 4096}},
+            .debug_flag_ = false,
+        };
+
+        DX1Collector gc(cfg);
+        auto alloc = with_facet<AAllocator>::mkobj(&gc);
+        auto coll = with_facet<ACollector>::mkobj(&gc);
+
+        bool ok = CollectorTypeRegistry::instance().install_types(coll);
+        REQUIRE(ok);
+
+        obj<AGCObject> prim_gco = with_facet<AGCObject>::mkobj(&Primitives::s_mul_gco_gco_pm);
+        obj<AExpression,DConstant> fn_expr = DConstant::make(alloc, prim_gco);
+
+        obj<AGCObject> val1 = DFloat::box<AGCObject>(alloc, 3.0);
+        obj<AGCObject> val2 = DFloat::box<AGCObject>(alloc, 7.0);
+        obj<AExpression,DConstant> arg1 = DConstant::make(alloc, val1);
+        obj<AExpression,DConstant> arg2 = DConstant::make(alloc, val2);
+
+        TypeRef result_type = TypeRef::resolved(Reflect::require<double>());
+        auto apply_expr = DApplyExpr::make2(alloc, result_type, fn_expr, arg1, arg2);
+
+        REQUIRE(apply_expr.data()->n_args() == 2);
+    }
+
+    TEST_CASE("DApplyExpr-fn", "[expression2][DApplyExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "dapplyexpr_fn_test",
+            .arena_config_ = ArenaConfig{
+                .size_ = 8192,
+                .store_header_flag_ = true},
+            .object_types_z_ = 16384,
+            .gc_trigger_v_{{4096, 4096}},
+            .debug_flag_ = false,
+        };
+
+        DX1Collector gc(cfg);
+        auto alloc = with_facet<AAllocator>::mkobj(&gc);
+        auto coll = with_facet<ACollector>::mkobj(&gc);
+
+        bool ok = CollectorTypeRegistry::instance().install_types(coll);
+        REQUIRE(ok);
+
+        obj<AGCObject> prim_gco = with_facet<AGCObject>::mkobj(&Primitives::s_mul_gco_gco_pm);
+        obj<AExpression,DConstant> fn_expr = DConstant::make(alloc, prim_gco);
+
+        obj<AGCObject> val1 = DFloat::box<AGCObject>(alloc, 3.0);
+        obj<AGCObject> val2 = DFloat::box<AGCObject>(alloc, 7.0);
+        obj<AExpression,DConstant> arg1 = DConstant::make(alloc, val1);
+        obj<AExpression,DConstant> arg2 = DConstant::make(alloc, val2);
+
+        TypeRef result_type = TypeRef::resolved(Reflect::require<double>());
+        auto apply_expr = DApplyExpr::make2(alloc, result_type, fn_expr, arg1, arg2);
+
+        // verify fn() returns an expression
+        obj<AExpression> fn = apply_expr.data()->fn();
+        REQUIRE(fn.data() != nullptr);
+        REQUIRE(fn.extype() == xo::scm::exprtype::constant);
+    }
+
+    TEST_CASE("DApplyExpr-arg", "[expression2][DApplyExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "dapplyexpr_arg_test",
+            .arena_config_ = ArenaConfig{
+                .size_ = 8192,
+                .store_header_flag_ = true},
+            .object_types_z_ = 16384,
+            .gc_trigger_v_{{4096, 4096}},
+            .debug_flag_ = false,
+        };
+
+        DX1Collector gc(cfg);
+        auto alloc = with_facet<AAllocator>::mkobj(&gc);
+        auto coll = with_facet<ACollector>::mkobj(&gc);
+
+        bool ok = CollectorTypeRegistry::instance().install_types(coll);
+        REQUIRE(ok);
+
+        obj<AGCObject> prim_gco = with_facet<AGCObject>::mkobj(&Primitives::s_mul_gco_gco_pm);
+        obj<AExpression,DConstant> fn_expr = DConstant::make(alloc, prim_gco);
+
+        obj<AGCObject> val1 = DFloat::box<AGCObject>(alloc, 3.0);
+        obj<AGCObject> val2 = DFloat::box<AGCObject>(alloc, 7.0);
+        obj<AExpression,DConstant> arg1 = DConstant::make(alloc, val1);
+        obj<AExpression,DConstant> arg2 = DConstant::make(alloc, val2);
+
+        TypeRef result_type = TypeRef::resolved(Reflect::require<double>());
+        auto apply_expr = DApplyExpr::make2(alloc, result_type, fn_expr, arg1, arg2);
+
+        // verify arg(0) and arg(1) return expressions
+        obj<AExpression> a0 = apply_expr.data()->arg(0);
+        obj<AExpression> a1 = apply_expr.data()->arg(1);
+
+        REQUIRE(a0.data() != nullptr);
+        REQUIRE(a1.data() != nullptr);
+        REQUIRE(a0.extype() == xo::scm::exprtype::constant);
+        REQUIRE(a1.extype() == xo::scm::exprtype::constant);
+    }
+
+    TEST_CASE("DApplyExpr-pretty", "[expression2][DApplyExpr][pp]")
+    {
+        scope log(XO_DEBUG(false));
+
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "dapplyexpr_pretty_test",
+            .arena_config_ = ArenaConfig{
+                .size_ = 8192,
+                .store_header_flag_ = true},
+            .object_types_z_ = 16384,
+            .gc_trigger_v_{{4096, 4096}},
+            .debug_flag_ = false,
+        };
+
+        DX1Collector gc(cfg);
+        auto alloc = with_facet<AAllocator>::mkobj(&gc);
+        auto coll = with_facet<ACollector>::mkobj(&gc);
+
+        bool ok = CollectorTypeRegistry::instance().install_types(coll);
+        REQUIRE(ok);
+
+        obj<AGCObject> prim_gco = with_facet<AGCObject>::mkobj(&Primitives::s_mul_gco_gco_pm);
+        obj<AExpression,DConstant> fn_expr = DConstant::make(alloc, prim_gco);
+
+        obj<AGCObject> val1 = DFloat::box<AGCObject>(alloc, 3.0);
+        obj<AGCObject> val2 = DFloat::box<AGCObject>(alloc, 7.0);
+        obj<AExpression,DConstant> arg1 = DConstant::make(alloc, val1);
+        obj<AExpression,DConstant> arg2 = DConstant::make(alloc, val2);
+
+        TypeRef result_type = TypeRef::resolved(Reflect::require<double>());
+        auto apply_expr = DApplyExpr::make2(alloc, result_type, fn_expr, arg1, arg2);
+
+        std::stringstream ss;
+        ppconfig ppc;
+        ppstate_standalone pps(&ss, 0, &ppc);
+
+        obj<APrintable,DApplyExpr> expr_pr(apply_expr.data());
+        pps.pretty(expr_pr);
+
+        std::string output = ss.str();
+
+        log && log(output);
+
+        CHECK(output.find("ApplyExpr") != std::string::npos);
+    }
+}
+
+/* end DApplyExpr.test.cpp */