A genetic analysis using enzyme data of 72 Leishmania mexicana isolated from hosts in Texas, Latin America, and the Caribbean is presented. All isolates from each country were combined and considered as local populations. Allomorph (allele determined by electrophoresis) frequencies for 20 enzyme (loci) were calculated and 7 populations (Texas, Mexico, Belize, Guatemala, Ecuador [EC], Venezuela, and the Dominican Republic [DR]) were compared pairwise in the statistic of genetic identity (I) (level of genetic similarity). All populations were found to be genetically similar with a mean I value for all comparisons of 0.890 ± 0.067. When DR was included as one of the pair compared, Ī = 0.811 ± 0.034. Among comparisons that include EC (excluding EC vs. DR), Ī = 0.875 ± 0.026. The mean I for the other comparisons was >0.9. The data indicate that the DR population is divergent enough from the others that it can be considered at the subspecies/incipient species level of evolutionary divergence; the EC population is, to a lesser extent, distinct from the others, and the other 5 represent geographic populations of 1 widely distributed species. A diagrammatic representation of the allomorphs among the 72 isolates is included. There were some allomorph/geographical (or local) population relationships noted.