PYTHIA was originally written in FORTRAN 77, until the 2007 release of PYTHIA 8.1 which was rewritten in C++. Both the Fortran and C++ versions were maintained until 2012 because not all components had been merged into the 8.1 version.
PYTHIA began as a FORTRAN 77 program but was later reworked in C++ with the release of PYTHIA 8.1 in 2007. The original Fortran and revised C++ versions were maintained until 2012, when not all components had been merged into the new version.