The genetics of two laboratory colonies of Glossina palpalis gambiensis were characterized by C-banding and isoenzyme studies. The colonies, derived from flies collected in the same locality, had different histories in the laboratory and different susceptibilities to trypanosome infection. Although the two lines were also found to differ in the frequencies of chromosome and isozyme variants, the variation was not enough to put their specific status in doubt; it was probably the result of genetic drift since the foundation of the colonies.