ELECTROPHORESIS has been used extensively for characterising genetic variability within and between natural populations1,2 but has not been applied to sympatric ecotypes. In the mosquito Aedes aegypti a dark feral form predominates throughout sub-Saharan Africa, but elsewhere in the tropics and moist subtropics a paler domestic form is found. The two ecotypes occur together in Africa in areas with a dry season during which natives store water in pots inside huts3,4. The indoor ecotype breeds continually in water stored in human dwellings, and the outdoor ecotype lives in treeholes and other sites around the dwellings and breeds seasonally. Our data provide the first demonstration of electrophoretic differences between sympatric ecotypes.