diff --git a/CMakeLists.txt b/CMakeLists.txt
index 773d5115..281407f4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -79,13 +79,15 @@ add_subdirectory(xo-ratio)
 add_subdirectory(xo-unit)
 add_subdirectory(xo-pyunit)
 #
+add_subdirectory(xo-callback)
+#
 add_subdirectory(xo-expression)
 add_subdirectory(xo-pyexpression)
 add_subdirectory(xo-tokenizer)
 add_subdirectory(xo-reader)
 add_subdirectory(xo-jit)
 add_subdirectory(xo-pyjit)
-#add_subdirectory(xo-callback)
+#
 #add_subdirectory(xo-webutil)
 #add_subdirectory(xo-pywebutil)
 #add_subdirectory(xo-printjson)
diff --git a/default.nix b/default.nix
index f3d02c30..ec39b4ca 100644
--- a/default.nix
+++ b/default.nix
@@ -51,7 +51,9 @@ let
           xo-ratio          = self.callPackage pkgs/xo-ratio.nix          {};
           xo-unit           = self.callPackage pkgs/xo-unit.nix           {};
           xo-pyunit         = self.callPackage pkgs/xo-pyunit.nix         {};
-
+#
+          xo-callback       = self.callPackage pkgs/xo-callback.nix       {};
+#
           xo-expression     = self.callPackage pkgs/xo-expression.nix     {};
           xo-pyexpression   = self.callPackage pkgs/xo-pyexpression.nix   {};
           xo-tokenizer      = self.callPackage pkgs/xo-tokenizer.nix      {};
@@ -61,7 +63,6 @@ let
                                                                             clang = llvmPackages.clang;
                                                                             llvm = llvmPackages.llvm; };
           xo-pyjit          = self.callPackage pkgs/xo-pyjit.nix          {};
-#          xo-callback       = self.callPackage pkgs/xo-callback.nix       {};
 #          xo-webutil        = self.callPackage pkgs/xo-webutil.nix        {};
 #          xo-pywebutil      = self.callPackage pkgs/xo-pywebutil.nix      {};
 #          xo-printjson      = self.callPackage pkgs/xo-printjson.nix      {};
diff --git a/pkgs/xo-callback.nix b/pkgs/xo-callback.nix
new file mode 100644
index 00000000..92cee250
--- /dev/null
+++ b/pkgs/xo-callback.nix
@@ -0,0 +1,28 @@
+{
+  # nixpkgs dependencies
+  stdenv, cmake, catch2,
+
+  # xo dependencies
+  xo-cmake,
+#  xo-refcnt,
+#  xo-reflect,
+    #, xo-indentlog,
+} :
+
+stdenv.mkDerivation (finalattrs:
+  {
+    name = "xo-callback";
+
+    src = ../xo-callback;
+
+    cmakeFlags = ["-DCMAKE_MODULE_PATH=${xo-cmake}/share/cmake"];
+    doCheck = true;
+    nativeBuildInputs = [
+      cmake catch2 xo-cmake
+    ];
+    propagatedBuildInputs = [
+#      xo-refcnt
+#      xo-reflect
+#      xo-indentlog
+    ];
+  })
diff --git a/pkgs/xo-userenv.nix b/pkgs/xo-userenv.nix
index c7bdd032..8591740a 100644
--- a/pkgs/xo-userenv.nix
+++ b/pkgs/xo-userenv.nix
@@ -36,10 +36,10 @@
   xo-ratio,
   xo-unit,
   xo-pyunit,
-
+#
+  xo-callback,
 #  xo-printjson,
 #  xo-pyprintjson,
-#  xo-callback,
 #  xo-webutil,
 #  xo-pywebutil,
 #  xo-reactor,
@@ -76,7 +76,9 @@ buildEnv {
             xo-ratio
             xo-unit
             xo-pyunit
-
+#
+            xo-callback
+#
             xo-expression
             xo-pyexpression
             xo-tokenizer
@@ -84,7 +86,6 @@ buildEnv {
             xo-jit
 
 #            xo-pyjit
-#            xo-callback
 #            xo-webutil
 #            xo-pywebutil
 #            xo-printjson