diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -22,6 +22,7 @@ This document follows the conventions laid out in [Keep a CHANGELOG][].
     attribute (#481)
 -   Fixed conversion of 'float' and 'double' values (#486)
 -   Fixed 'clrmethod' for python 2 (#492)
+-   Fixed double calling of constructor when deriving from .NET class (#495)
 
 
 ## [2.3.0][] - 2017-03-11

diff --git a/src/runtime/classderived.cs b/src/runtime/classderived.cs
@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
@@ -55,6 +55,33 @@ internal ClassDerivedObject(Type tp) : base(tp)
             return Converter.ToPython(obj, cls.GetType());
         }
 
+        public new static int tp_init(IntPtr obj, IntPtr args, IntPtr kw)
+        {
+            Runtime.XIncref(obj);
+            Runtime.XIncref(Runtime.PyNone);
+            using (var pyself = new PyObject(obj))
+            using (var pynone = new PyObject(Runtime.PyNone))
+            using (var init = pyself.GetAttr("__init__", pynone))
+            {
+                if (init.Handle != Runtime.PyNone)
+                {
+                    // if __init__ hasn't been overridden then it will be a managed object
+                    if (GetManagedObject(init.Handle) == null)
+                    {
+                        Runtime.XIncref(args);
+                        Runtime.XIncref(kw);
+                        using (var args_ = new PyTuple(args))
+                        using (var kw_ = new PyDict(kw))
+                        using (var res = init.Invoke(args_, kw_))
+                        {
+                        }
+                    }
+                }
+            }
+
+            return 0;
+        }
+
         public new static void tp_dealloc(IntPtr ob)
         {
             var self = (CLRObject)GetManagedObject(ob);
@@ -822,33 +849,7 @@ public static void InvokeCtor(IPythonDerivedType obj, string origCtorName, objec
                 FieldInfo fi = obj.GetType().GetField("__pyobj__");
                 fi.SetValue(obj, self);
 
-                Runtime.XIncref(self.pyHandle);
-                var pyself = new PyObject(self.pyHandle);
-                disposeList.Add(pyself);
-
-                Runtime.XIncref(Runtime.PyNone);
-                var pynone = new PyObject(Runtime.PyNone);
-                disposeList.Add(pynone);
-
-                // call __init__
-                PyObject init = pyself.GetAttr("__init__", pynone);
-                disposeList.Add(init);
-                if (init.Handle != Runtime.PyNone)
-                {
-                    // if __init__ hasn't been overridden then it will be a managed object
-                    ManagedType managedMethod = ManagedType.GetManagedObject(init.Handle);
-                    if (null == managedMethod)
-                    {
-                        var pyargs = new PyObject[args.Length];
-                        for (var i = 0; i < args.Length; ++i)
-                        {
-                            pyargs[i] = new PyObject(Converter.ToPython(args[i], args[i]?.GetType()));
-                            disposeList.Add(pyargs[i]);
-                        }
 
-                        disposeList.Add(init.Invoke(pyargs));
-                    }
-                }
             }
             finally
             {

diff --git a/src/runtime/metatype.cs b/src/runtime/metatype.cs
@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Runtime.InteropServices;
 
 namespace Python.Runtime
@@ -125,19 +125,6 @@ public static IntPtr tp_new(IntPtr tp, IntPtr args, IntPtr kw)
         }
 
 
-        public static IntPtr tp_alloc(IntPtr mt, int n)
-        {
-            IntPtr type = Runtime.PyType_GenericAlloc(mt, n);
-            return type;
-        }
-
-
-        public static void tp_free(IntPtr tp)
-        {
-            Runtime.PyObject_GC_Del(tp);
-        }
-
-
         /// <summary>
         /// Metatype __call__ implementation. This is needed to ensure correct
         /// initialization (__init__ support), because the tp_call we inherit
@@ -188,6 +175,19 @@ public static IntPtr tp_call(IntPtr tp, IntPtr args, IntPtr kw)
         }
 
 
+
+        public static IntPtr tp_alloc(IntPtr mt, int n)
+        {
+            IntPtr type = Runtime.PyType_GenericAlloc(mt, n);
+            return type;
+        }
+
+
+        public static void tp_free(IntPtr tp)
+        {
+            Runtime.PyObject_GC_Del(tp);
+        }
+
         /// <summary>
         /// Type __setattr__ implementation for reflected types. Note that this
         /// is slightly different than the standard setattr implementation for

diff --git a/src/tests/test_subclass.py b/src/tests/test_subclass.py
@@ -190,3 +190,17 @@ def test_isinstance_check():
     for x in b:
         assert isinstance(x, System.Object)
         assert isinstance(x, System.String)
+
+
+def test_derived_constructor_only_called_once():
+    calls = []
+
+    class X(System.Object):
+        __namespace__ = "PyTest"
+
+        def __init__(self, *args):
+            calls.append(args)
+
+    x = X(1, 2)
+
+    assert calls == [(1, 2)]