ASA109 is a FORTRAN77 library which "inverts" the incomplete Beta function; that is, given a value of the incomplete Beta function, it can determine the argument that produced that value.
ASA109 is Applied Statistics Algorithm 109. Source code for many Applied Statistics Algorithms is available through STATLIB.
ASA109 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.
ASA063, a FORTRAN77 library which evaluates the incomplete Beta function.
ASA226, a FORTRAN77 library which evaluates the CDF of the noncentral Beta distribution.
ASA310, a FORTRAN77 library which computes the CDF of the noncentral Beta distribution.
BETA_NC, a FORTRAN90 library which evaluates the CDF of the noncentral Beta distribution.
TEST_VALUES, a FORTRAN77 library which stores a few values of various mathematical functions.
TOMS179, a FORTRAN77 library which evaluates the incomplete Beta function.
TOMS708, a FORTRAN77 library which evaluates the incomplete Beta function.
You can go up one level to the FORTRAN77 source codes.