Genetic variability was examined in two endemic cichlid fishes from Cuatro Cienegas, Coahuila, Mexico, and in Cichlasoma cyanoguttatum from the Rio Grande in Texas. The Mexican cichlids exhibit morphological divergence, endemism, and sympatry that is strikingly similar to that observed in cichlids of the African lake species-flocks. The endemic species occur in restricted waters that are relatively thermally constant. C. cyanoguttatum is widespread and can be regarded as occurring in a relatively variable environment. A comparison of the levels of genetic variability among species was proposed as a test of the generally recognized relationship between environmental heterogeneity and genetic variability. A minimum of thirteen electrophoretic loci were studied in all species. C. cyanoguttatum was monomorphic at all loci in a sample of 38 individuals. The endemic detritus-eating cichlid (N = 79) and endemic snail-eating cichlid (N = 39) possessed 3.63 and 3.43% heterozygous loci per individual, respectively. These levels of heterozygosity are low relative to other vertebrates, and are not consistent with interpretations of inbreeding or founder effects. Moreover, the occurrence of monomorphism in a 'variable' environment causes us to question the generality of a relation between environmental heterogeneity and genetic variability.