diff --git a/xo-expression2/utest/CMakeLists.txt b/xo-expression2/utest/CMakeLists.txt
index b1662736..f464708e 100644
--- a/xo-expression2/utest/CMakeLists.txt
+++ b/xo-expression2/utest/CMakeLists.txt
@@ -8,6 +8,7 @@ set(UTEST_SRCS
     DConstant.test.cpp
     DVariable.test.cpp
     DApplyExpr.test.cpp
+    DDefineExpr.test.cpp
 )
 
 xo_add_utest_executable(${UTEST_EXE} ${UTEST_SRCS})
diff --git a/xo-expression2/utest/DDefineExpr.test.cpp b/xo-expression2/utest/DDefineExpr.test.cpp
new file mode 100644
index 00000000..5bb91582
--- /dev/null
+++ b/xo-expression2/utest/DDefineExpr.test.cpp
@@ -0,0 +1,317 @@
+/** @file DDefineExpr.test.cpp
+ *
+ *  @author Roland Conybeare, Jan 2026
+ **/
+
+#include "init_expression2.hpp"
+#include <xo/expression2/DDefineExpr.hpp>
+#include <xo/expression2/detail/IExpression_DDefineExpr.hpp>
+#include <xo/expression2/detail/IPrintable_DDefineExpr.hpp>
+#include <xo/expression2/DConstant.hpp>
+#include <xo/expression2/detail/IExpression_DConstant.hpp>
+#include <xo/expression2/StringTable.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>
+#include <cstring>
+
+namespace ut {
+    using xo::S_expression2_tag;
+    using xo::scm::DDefineExpr;
+    using xo::scm::DConstant;
+    using xo::scm::DFloat;
+    using xo::scm::DVariable;
+    using xo::scm::DUniqueString;
+    using xo::scm::StringTable;
+    using xo::scm::AExpression;
+    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("DDefineExpr-init", "[expression2][DDefineExpr]")
+    {
+        REQUIRE(s_init.evidence());
+    }
+
+    TEST_CASE("DDefineExpr-make", "[expression2][DDefineExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "ddefineexpr_make_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);
+
+        StringTable table(1024);
+        const DUniqueString * name = table.intern("x");
+        REQUIRE(name != nullptr);
+
+        // Create rhs expression: constant 42.0
+        obj<AGCObject> fval = DFloat::box<AGCObject>(alloc, 42.0);
+        auto rhs_expr = DConstant::make(alloc, fval);
+        REQUIRE(rhs_expr.data() != nullptr);
+
+        // Create define expression: def x = 42.0
+        DDefineExpr * def = DDefineExpr::make(alloc, name, rhs_expr);
+        REQUIRE(def != nullptr);
+    }
+
+    TEST_CASE("DDefineExpr-lhs", "[expression2][DDefineExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "ddefineexpr_lhs_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);
+
+        StringTable table(1024);
+        const DUniqueString * name = table.intern("myvar");
+
+        obj<AGCObject> fval = DFloat::box<AGCObject>(alloc, 3.14);
+        auto rhs_expr = DConstant::make(alloc, fval);
+
+        DDefineExpr * def = DDefineExpr::make(alloc, name, rhs_expr);
+        REQUIRE(def != nullptr);
+
+        DVariable * lhs = def->lhs();
+        REQUIRE(lhs != nullptr);
+        REQUIRE(lhs->name() == name);
+    }
+
+    TEST_CASE("DDefineExpr-rhs", "[expression2][DDefineExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "ddefineexpr_rhs_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);
+
+        StringTable table(1024);
+        const DUniqueString * name = table.intern("y");
+
+        obj<AGCObject> fval = DFloat::box<AGCObject>(alloc, 2.718);
+        auto rhs_expr = DConstant::make(alloc, fval);
+
+        DDefineExpr * def = DDefineExpr::make(alloc, name, rhs_expr);
+        REQUIRE(def != nullptr);
+
+        obj<AExpression> rhs = def->rhs();
+        REQUIRE(rhs.data() != nullptr);
+        // Verify rhs expression type is constant
+        REQUIRE(rhs.extype() == xo::scm::exprtype::constant);
+    }
+
+    TEST_CASE("DDefineExpr-name", "[expression2][DDefineExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "ddefineexpr_name_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);
+
+        StringTable table(1024);
+        const DUniqueString * name = table.intern("foo");
+
+        obj<AGCObject> fval = DFloat::box<AGCObject>(alloc, 1.0);
+        auto rhs_expr = DConstant::make(alloc, fval);
+
+        DDefineExpr * def = DDefineExpr::make(alloc, name, rhs_expr);
+        REQUIRE(def != nullptr);
+        REQUIRE(def->name() == name);
+        REQUIRE(std::strcmp(def->name()->chars(), "foo") == 0);
+    }
+
+    TEST_CASE("DDefineExpr-extype", "[expression2][DDefineExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "ddefineexpr_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);
+
+        StringTable table(1024);
+        const DUniqueString * name = table.intern("z");
+
+        obj<AGCObject> fval = DFloat::box<AGCObject>(alloc, 0.0);
+        auto rhs_expr = DConstant::make(alloc, fval);
+
+        DDefineExpr * def = DDefineExpr::make(alloc, name, rhs_expr);
+        REQUIRE(def != nullptr);
+        REQUIRE(def->extype() == xo::scm::exprtype::define);
+    }
+
+    TEST_CASE("DDefineExpr-valuetype", "[expression2][DDefineExpr]")
+    {
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "ddefineexpr_valuetype_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);
+
+        StringTable table(1024);
+        const DUniqueString * name = table.intern("w");
+
+        obj<AGCObject> fval = DFloat::box<AGCObject>(alloc, 99.9);
+        auto rhs_expr = DConstant::make(alloc, fval);
+
+        DDefineExpr * def = DDefineExpr::make(alloc, name, rhs_expr);
+        REQUIRE(def != nullptr);
+        REQUIRE(def->valuetype() == Reflect::require<double>());
+    }
+
+    TEST_CASE("DDefineExpr-pretty", "[expression2][DDefineExpr][pp]")
+    {
+        scope log(XO_DEBUG(true));
+
+        REQUIRE(s_init.evidence());
+
+        CollectorConfig cfg{
+            .name_ = "ddefineexpr_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);
+
+        StringTable table(1024);
+        const DUniqueString * name = table.intern("bar");
+
+        obj<AGCObject> fval = DFloat::box<AGCObject>(alloc, 123.456);
+        auto rhs_expr = DConstant::make(alloc, fval);
+
+        DDefineExpr * def = DDefineExpr::make(alloc, name, rhs_expr);
+        REQUIRE(def != nullptr);
+
+        std::stringstream ss;
+        ppconfig ppc;
+        ppstate_standalone pps(&ss, 0, &ppc);
+
+        obj<APrintable,DDefineExpr> def_pr(def);
+        pps.pretty(def_pr);
+
+        std::string output = ss.str();
+
+        log && log(output);
+
+        CHECK(output.find("DDefineExpr") != std::string::npos);
+    }
+}
+
+/* end DDefineExpr.test.cpp */