If the F77 files are really written in F77 then they should compile without a hitch. Some simple extensions (like IMPLICIT NONE) should also work just fine. Beyond that, all bets are off. First off, what does the Sun compiler say if you compile with -C and -ansi (to get all the checking turned on)? See if similar errors turn up to when you try compiling with g77. FWIW, what errors do you get on your OSX box?