THE existence of sympatric speciation-that populations diverge into species in the absence of physical or ecological barriers-is controversial1-6. The East African Great Lakes harbour hundreds of cichlid species representing only a few monophyletic lineages7,8, although palaeolimnological evidence9-11 and local restrictions on species distribution12 suggest that speciation in these lakes could have been allopatric13,14. The case for sympatry in restricted areas of Lakes Malawi and Tanganyika is stronger15-17 but not unassailable. A better case might be made for cichlid species flocks in small, ecologically monotonous crater lakes. Here we present a mitochondrial DNA analysis of cichlid species flocks endemic to two such lakes in Cameroon. The results suggest that the flocks in each lake are monophyletic : the implication being that each lake was colonized once only, the size and shape of each lake being such that subsequent diversification would have been sympatric.