Can animal mating systems result in the choice of mates carrying genotypes that are otherwise favored by natural selection? This question is addressed by studying, in natural populations of Colias butterflies, how the phosphoglucose isomerase (PGI) enzyme genotype of males mating Colias females varies with degree of female mate discrimination. Certain PGI genotypes (as predicted from their biochemical properties) have been found previously to have an advantage in diverse fitness-related properties: flight capacity, survivorship, and overall mating success. It is shown here that males of these same genotypes have even greater advantage in remating older, more discriminating females than they do in mating previously unmated, less discriminating females. Assortative mating is not found and thus cannot explain this effect. The mating system of these insects does, at least in this case, result in active female choice of generally favorable male genotypes as mates.