For g77 running on Linux the standard debugger is gdb (xxgdb) which has serious limitations. It does not display variables in COMMON easily, although a fix developed by the FINUDA group makes this possible. However it is awkward and not easy to use. The xfx debugger supplied with ABSOFT f90 seems to work very well. It interfaces to ABSOFT-compiled fortran and (apparently ) to gcc sources as well, and gives full access to global variables and structures and pointers. I have found the watch and monitor facilities particularly useful. To debug using the fortran sources one must specify the directory paths where they reside. This is done with the REINIT command or by starting xfx without arguements. Default starting values can be put in an .fxinit file.