I am having trouble using callback_exn -- my program crashes in bytecode mode, but works ok if I use the native-code compiler. I saw a message in the archive discussing a problem like this: http://caml.inria.fr/archives/200204/msg00065.html The CVS shows a change to byterun/interp.c on April 12 that appears to be related to this problem. I've tried to apply this change to my own copy of OCaml, but my program still crashes. Sometimes it runs a little bit longer than it did before, but it still crashes within the next few function calls. Was the April 12 fix supposed to resolve the problem completely? I'm enclosing a small program that demonstrates the problem. When prompted for a string, if you type "hello" the OCaml code will raise an exception, and the program will crash shortly thereafter. The timing of the crash varies depending whether you use "loop" or "loop1" as the main program loop. (I'm running OCaml 3.04, native Win32 port, on Windows 2000.)