Hey, I just realized I can shave off another 30% in C# ;-) So now the timings become: Safe Haskell ========= J:\dev\haskell>ghc -O2 -o primechaddai.exe PrimeChaddai.hs J:\dev\haskell>primechaddai number of primes: 664579 Elapsed time: 26.234 Unsafe Haskell =========== J:\dev\haskell>ghc -fglasgow-exts -O2 -o PrimeDonald2.exe PrimeDonald2.hs J:\dev\haskell>primedonald2 number of primes: 664579 Elapsed time: 0.7030000000000001 mono ==== J:\dev\test\testperf>erase primecs.exe & gmcs primecs.cs J:\dev\test\testperf>mono primecs.exe number of primes: 664579 elapsed time: 0,453 Microsoft.Net ========== J:\dev\test\testperf>erase primecs.exe & csc /nologo primecs.cs J:\dev\test\testperf>primecs number of primes: 664579 elapsed time: 0,421875 Here's the fabulously complicated ;-) new C# algorithm: public int CalculateNumberOfPrimes( int maxprime ) { bool[]IsNotPrime = new bool[ maxprime ]; int NumberOfPrimes = 1; int squarecutoff = (Int32)Math.Sqrt( maxprime ) + 1; for( int i = 3; i < maxprime; i+= 2 ) { if( !IsNotPrime [i] ) { NumberOfPrimes++; if( i < squarecutoff ) { for( int j = ( i << 1 ); j < maxprime; j+= i ) { if( !IsNotPrime [j] ) IsNotPrime [ j] = true; } } } } return NumberOfPrimes; } (basically, we only cross off primes up to the square root of the size of the grid. I think this is a standard part of the standard Arostophenes grid algorithm, so like I say the C# version is seriously unoptimized ;-) )