Thirty-four populations of Aedes aegypti representing the world-wide distribution of the species, were analysed for genetic variation at 19–22 isozyme loci. The species has an average expected heterozygosity of 0·129±0·045 based on 19 loci analysed in every population. Based on this genetic information, two major groups can be defined: the dark, often sylvan, African subspecies formosus and the light domestic subspecies aegypti in Africa and the New World. Asian populations do not fall easily into either group. These results are related to models which have been proposed for the evolution of this species. Although A. aegypti was introduced into the New World c. 350 years ago and has recently recolonized many areas following eradication programmes, no signs of founder effects are evident in this region. Asian populations, on the other hand, do show a significantly lower level of genetic variation compared to other populations. This may be related to the time of introduction of A. aegypti into Asia and historical absence of yellow fever on the Asian continent.