Isozyme patterns of 13 field-collected populations of Aedes aegypti from Thailand were compared using starch gel electrophoresis. Three populations were collected before the Bacillus thuringiensis var. israelensis, (B.t.i.) application was initiated. The other 10 populations were collected after the B.t.i. treatment. Results revealed that the number of polymorphic loci were lower in the B.t.i. treated populations as compared to controls. In addition, lower genetic variability was found in populations collected from B.t.i. treated sites (Mae Ka Sa [KS] and Mae Kud Luang [KL] village). These results are most likely due to a genetic bottleneck produced by the B.t.i. treatment. Heterozygosity increased in the months following B.t.i. treatment, probably because of immigration when the control program was withdrawn. However, the anticipated reduction in the expected heterozygosity was only observed in the KS site. This may be due to preexisting low heterozygosity in the KL population. No fixed differences in alleles were detected among the 13 populations.