Estimates of allele frequencies at six polymorphic loci were collected over eight generations in two populations of Euphydryas editha. We have estimated, in addition, the effective population size for each generation for both populations with results from mark-recapture and other field data. The variation in allele frequencies generated by random genetic drift was then studied using computer simulations and our direct estimates of effective population size. Substantial differences between observed values and computer-generated expected values assuming drift alone were found for three loci (Got, Hk, Pgi) in one population. These observations are consistent with natural selection in a variable environment.