@Julien I have been calling Python from C# without any problems even in a multi-threaded setup. I am using .Net 2.0 and Python 2.5. Main difference, at least from what I don't see in your code, is I also call BeginAllowThreads() after Initialize(). This allows me to initialize PythonEngine in a different thread once and never have to worry about it again. I remember having similar problem when I first embedded Python and this doc helped me come up with this solution. http://docs.python.org/c-api/init.html#thread-state-and-the-global-interpret... BTW, AcquireLock() below always returns 1. Please let me know if this helps. /* This is in main thread */ if (!PythonEngine.IsInitialized) { PythonEngine.Initialize(); PythonEngine.BeginAllowThreads(); } /* This is ran in another thread */ IntPtr gs = PythonEngine.AcquireLock(); PyObject module = PythonEngine.ImportModule(pymodule); try { PyObject res = module.InvokeMethod(method_name, _args); /* Python calls here */ .... } catch (PythonException pe) { /* Handle python exceptions */ } finally { PythonEngine.ReleaseLock(gs); }