diff --git a/CMakeLists.txt b/CMakeLists.txt
index d4846789..aeb45c9e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -97,6 +97,7 @@ add_subdirectory(xo-pysimulator)
 add_subdirectory(xo-process)
 add_subdirectory(xo-pyprocess)
 add_subdirectory(xo-kalmanfilter)
+add_subdirectory(xo-pykalmanfilter)
 #
 add_subdirectory(xo-expression)
 add_subdirectory(xo-pyexpression)
@@ -105,4 +106,3 @@ add_subdirectory(xo-reader)
 add_subdirectory(xo-jit)
 add_subdirectory(xo-pyjit)
 #
-#add_subdirectory(xo-pykalmanfilter)
diff --git a/default.nix b/default.nix
index 660f3a7b..dd3acef7 100644
--- a/default.nix
+++ b/default.nix
@@ -70,6 +70,7 @@ let
           xo-process        = self.callPackage pkgs/xo-process.nix        {};
           xo-pyprocess      = self.callPackage pkgs/xo-pyprocess.nix      {};
           xo-kalmanfilter   = self.callPackage pkgs/xo-kalmanfilter.nix   {};
+          xo-pykalmanfilter = self.callPackage pkgs/xo-pykalmanfilter.nix {};
 #
           xo-expression     = self.callPackage pkgs/xo-expression.nix     {};
           xo-pyexpression   = self.callPackage pkgs/xo-pyexpression.nix   {};
@@ -81,7 +82,6 @@ let
                                                                             llvm = llvmPackages.llvm; };
           xo-pyjit          = self.callPackage pkgs/xo-pyjit.nix          {};
 
-#          xo-pykalmanfilter = self.callPackage pkgs/xo-pykalmanfilter.nix {};
 #
           xo-userenv        = self.callPackage pkgs/xo-userenv.nix        {};
 #          xo-userenv-slow   = self.callPackage pkgs/xo-userenv-slow.nix   {};
diff --git a/pkgs/xo-pykalmanfilter.nix b/pkgs/xo-pykalmanfilter.nix
new file mode 100644
index 00000000..5a324720
--- /dev/null
+++ b/pkgs/xo-pykalmanfilter.nix
@@ -0,0 +1,41 @@
+{
+  # nixpkgs dependencies
+  stdenv, cmake, catch2, # ... other deps here
+
+  # xo dependencies
+  xo-cmake,
+  xo-kalmanfilter,
+
+  xo-pyutil,
+  xo-pyreactor,
+
+  # args
+
+  #   attrset for fetching source code.
+  #    { type, owner, repo, ref }
+  #
+  #   e.g. type="github", owner="rconybea", repo="cmake-examples", ref="ex1b"
+  #
+  #   see [[../flake.nix]]
+  #
+  #cmake-examples-ex1-path
+
+  # someconfigurationoption ? false
+} :
+
+stdenv.mkDerivation (finalattrs:
+  {
+    name = "xo-pykalmanfilter";
+    version = "1.0";
+
+    src = ../xo-pykalmanfilter;
+
+    cmakeFlags = ["-DCMAKE_MODULE_PATH=${xo-cmake}/share/cmake"];
+    doCheck = true;
+    nativeBuildInputs = [ cmake catch2 xo-cmake ];
+    propagatedBuildInputs = [
+      xo-pyutil
+      xo-pyreactor
+      xo-kalmanfilter
+    ];
+  })
diff --git a/pkgs/xo-userenv.nix b/pkgs/xo-userenv.nix
index 7728deac..b3916af9 100644
--- a/pkgs/xo-userenv.nix
+++ b/pkgs/xo-userenv.nix
@@ -55,7 +55,7 @@
   xo-process,
   xo-pyprocess,
   xo-kalmanfilter,
-  #  xo-pykalmanfilter,
+  xo-pykalmanfilter,
 #
   xo-expression,
   xo-pyexpression,
@@ -95,6 +95,7 @@ buildEnv {
             xo-pyreactor
             xo-websock
             xo-pywebsock
+#
             xo-statistics
             xo-distribution
             xo-pydistribution
@@ -102,6 +103,8 @@ buildEnv {
             xo-pysimulator
             xo-process
             xo-pyprocess
+            xo-kalmanfilter
+            xo-pykalmanfilter
 #
             xo-expression
             xo-pyexpression
@@ -109,7 +112,5 @@ buildEnv {
             xo-reader
             xo-jit
             xo-pyjit
-            xo-kalmanfilter
-#            xo-pykalmanfilter
           ];
 }