Populations of the specialist herbivore, Euphilotes enoptes (Lepidoptera: Lycaenidae), along three elevational transects in the. mountains of central Washington state, differed markedly in the phenology of adult flight. In spite of this apparent limitation to gene flow, six allozyme loci revealed substantial gene exchange among populations along these gradients. The elevational difference, and thus the phenological difference, between populations has not influenced the extent of gene flow between them. Because the direct exchange of genes between low- and high-elevation populations is very unlikely, gene flow between them has probably occurred in a stepwise fashion via intermediate populations. It is hypothesized that such gene flow has been biased in an uphill direction due to the combined effects of source size and oviposition behavior. Adult emergence times of populations in the same region are positively correlated with elevation in a nonlinear fashion, consistent with the hypothesis that gene flow from low-elevation populations has been swamping selection at higher altitudes.