Culiseta melanura (Coq.), the enzootic vector of eastern equine encephalitis in North America, is polymorphic for a trait used as a key diagnostic character. The absence of white abdominal bands distinguishes this species in several prominent keys to North American mosquitoes. However, this is an environmentally induced, nongenetic trait that cannot be used as a key character for diagnosing Cs. melanura. In light trap collections, banded specimens occur in early spring and summer, and nonbanded adults appear in late summer-autumn. Larvae reared in laboratory conditions produce nonbanded adults. Progeny reared from banded mothers are uniformly nonbanded. Biochemical genetic results indicate that banding is not correlated with a distinctive genotype or presence of cryptic species. In 18 enzyme loci screened, neither diagnostic alleles nor large differences in allele frequencies were detected between field-collected representatives of the two forms. Genetic variability was relatively low in the 28-year-old laboratory colony (average heterozygosity = 7%; average number of alleles per locus = 1.4), whereas in field samples, the variability was typical of field populations (average heterozygosity = 12-19%; average number of alleles per locus = 1.6-1.8), with the presence of both polymorphic and private alleles. The population genetic profile and comparisons among geographically distinct populations represent the first such presentation for any species in the genus Culiseta.