A comparison of electrophoretically detectable isozyme differences in 6 populations of Anopheles quadrimaculatus (sensu stricto) from northeastern Arkansas was undertaken to test the hypothesis that microgeographic variation in habitat types was promoting significant within- and between-population genetic diversity. Genetic heterogeneity within populations was substantial, with all of the enzyme loci examined having 2-7 alleles and average levels of polymorphisms per population between 54.5 and 72.7%. Heterozygotes made up an average over all loci of between 20.6 and 24.8% of the individuals examined. Only weak evidence was found for gametic disequilibrium between pairs of loci. Neither F-statistic nor genetic distance analysis suggested interpopulation divergence. The FST value averaged over loci was 0.190. All Nei distances for pair-wise population comparisons were greater than 0.010, which was much lower than published values from comparisons between populations belonging to different species of the complex. Divergence was not significantly correlated to either geographic distance or habitat type. Examination of the results suggests that little genetic divergence has occurred between populations of An. quadrimaculatus in northeastern Arkansas, possibly because of the dispersal ability and low level of discrimination between oviposition sites exhibited by this species.