ASA109 is a FORTRAN77 program, using double precision arithmetic, 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.
ASA063 a FORTRAN77 library which evaluates the incomplete Beta function.
ASA109 is also available in a FORTRAN90 version.
ASA226 is a FORTRAN77 library which evaluates the CDF of the noncentral Beta distribution.
ASA310 is a FORTRAN77 library which computes the CDF of the noncentral Beta distribution.
BETA_NC is a FORTRAN90 library which evaluates the CDF of the noncentral Beta distribution.
TEST_VALUES is a FORTRAN77 library which stores a few values of various mathematical functions.
TOMS179 is a FORTRAN77 library which evaluates the incomplete Beta function.
TOMS708 is a FORTRAN77 library which evaluates the incomplete Beta function.
You can go up one level to the FORTRAN77 source codes.