A dimeric esterase of a lycaenid butterfly shows rich, electrophoretically-detectable variation in natural populations in Texas. This isozyme variation is controlled by multiple alleles at an autosomal locus (Es-d). In each population sampled there are 9-14 alleles, two of which (Es-d(100) and Es-d(80)) predominate and form about 84% of the gene pool. Against the background of numerous rarer alleles, their frequencies are notably stable (about 65% and 19%, respectively) in ecologically variable space and time. Although adults (especially females) are capable of genetically effective dispersal, environmental heterogeneity within each locality is thought to be the prime factor maintaining the polymorphism.