After installing OS X El Capitan on my computer I noticed that fsharp interactive failed to start because it failed to locate mono: /Library/Frameworks/Mono.framework/Versions/4.0.4/bin/fsharpi: line 24: exec: mono: not found Session termination detected. Press Enter to restart. If we look at the file at that position we find that it assumes that mono is in the path. I checked /etc/paths, and it looked correct (i.e. based on the paths file, mono should have been in the path): mathieu$ which mono /usr/[...]