Thank, Daniel Multiple threads are in evidence in my system monitor, but I wonder why I'm getting two different answers, one twice the other. The first is the parallel solution and the second is the non.

Michael =========== {- import Control.Parallel nfib :: Int -> Int nfib n | n <= 1 = 1        | otherwise = par n1 (pseq n2 (n1 + n2 + 1))                      where n1 = nfib (n-1)                            n2 = nfib (n-2) -} nfib :: Int -> Int nfib n | n <= 1 = 1        | otherwise = nfib (n-1) + nfib (n-2) main = do putStrLn $ show $ nfib 39 ============= [michael@hostname ~]$ ghc --make -threaded nfib.hs [1 of 1] Compiling Main             ( nfib.hs, nfib.o ) Linking nfib ... [michael@hostname ~]$ ./nfib +RTS -N3 204668309 [michael@hostname ~]$ ghc --make nfib.hs [1 of 1] Compiling Main             ( nfib.hs, nfib.o ) Linking nfib ... [michael@hostname ~]$ ./nfib 102334155 [michael@hostname ~]$