Fortran 90/95 is in some implementations difficult to mix with Fortran 77. A discussion of this is given in chapter 15 of the tutorial by Bo Einarsson and Yurij Shokin.
A recent development is Fortran 2003, which has facilities for mixing Fortran and C in a standardized way, see my draft article Mixed Language Programming, Fortran 2003 and C.
Several papers on mixed language programming have been published. A Bibliography on Mixed Language Programming is available, but it ought to be updated.