Hi, Following on from this posting: http://lists.boost.org/boost-users/2007/02/25361.php I am having the same problem with program_options using std::string, compiling my code with gcc-4.0, using Xcode 2.2.1 on Mac OS X 10.4.9. Forcing -fvisibility=default onto the command line does not fix the problem. Do I need to rebuild the libraries with this flag (and if so, how?), or can somebody offer any other suggestions? Regards, James Jackson.