I have F# 2.0.0 installed, and use mono 2.8.
let rec fib n = match n with | 1 | 2 -> 1 | n -> fib(n-1) + fib(n-2) let n = 40 let x = fib(n
The F# assemblies (like FSharp.Core.dll) are probably only installed to the .Net GAC. You need to install them to Mono's GAC (Mono's gacutil.exe), or copy the assemblies into the same directory as your executable.