Electrophoretically detectable isozyme differences in 15 populations of Anopheles quadrimaculatus (Say) (sensu stricto) from eastern Arkansas were compared to measure levels of genetic diversity and study the sources of the variation. All of the enzyme loci had 2–7 alleles. Average levels of polymorphism per population were 88.9%. Heterozygotes for alleles of at least 1 of the 9 loci made up an average overall loci of 0.323 ± 0.078 of the individuals examined. F-statistic analysis suggested a small, but statistically significant interpopulation differentiation of heterozygote frequency. The reduced heterozygote frequency was not attributable to the presence of more than one species in any population nor to the preferential use of oviposition habitats by certain populations within the species. Nei’s distance values for pairwise population comparisons were small (<0.06). Correlation between genetic and geographic distance matrices was not significant. Migration among populations in the agricultural areas of the Arkansas delta region is apparently sufficient to homogenize most of the genetic divergence arising because of habitat or geographic isolation between populations in the region.