Evolutionary and ecological effects of DNA and protein polymorphisms are controversially discussed, since the raising of molecular approaches. Functional differences among different genotypes of allozymes might strongly influence evolutionary processes and spatial distributions. Therefore, we analysed nine polymorphic allozyme loci for 127 individuals sampled from one population of the chalk-hill blue Polyommatus coridon during one flight season. We observed fluctuations in the genotype frequencies during this flight season, but only two enzyme loci showed significant changes. The percentage of observed heterozygosity was decreasing in more than three quarters of all loci over time (e.g. Mdh2 decreasing from 72.7% to 36.4%). These changes might be due to different adaptations of heterozygotic individuals if compared with homozygotic ones. In contrast to genotypes, no changes in allele frequencies were observed (e.g. low FST value for Mdh2). Therefore, allozyme data represent a suitable tool for studying ecological adaptation and biogeographical pattern.