Demographically oriented sampling in the wild and biochemical study of allozymes in the laboratory have been used to probe maintenance of the phosphoglucose isomerase polymorphism of Colias butterflies.-The several alleles at this locus show negative or no covariation among their frequencies in the wild. This rules out Wahlund effects as a cause of observations of heterozygote excess at this locus in broods that fly as single cohorts. Unusually heavy mortality among adults, due to drought stress or other causes, can preclude manifestation of differential survivorship among phosphoglucose isomerase genotypes. In broods composed of overlapping cohorts, heterozygote deficiency, apparently due to Wahlund effects in time as cohorts of different survivorship experience mix, can be found. Heterozygotes at this locus fly under a broader range of weather conditions than other genotypes.-Previously detected kinetic differentiation among the genotypes extends in greater magnitude to the glycolytic reaction direction, as well as to a broader range of test conditions than examined before. The heterozygote 3/4 is strikingly heterotic for several measures of kinetic functional effectiveness. Other heterozygotes are sometimes heterotic, more often intermediate (but not exactly so, nor additive in any sense) in properties between homozygotes.-Predictions are made from the biochemical analysis and from the insects' thermal ecology concerning distributions of the genotypes in the wild. Some agree with facts already established. Others are tested and confirmed from data already on hand. Still others are to be tested as reported in an accompanying paper.-All available evidence points to a combination of heterozygote advantage and fluctuating-environment selection as responsible for maintaining this polymorphism. There is considerable evidence for the operation of protein-structural constraint on the range of adaptations possible at this locus.