Plague has been known from countries surrounding Rhodesia from as early as 1935, but was first reported from Rhodesia in 1974. Part of our investigation of the complex ecosystem involving Yersinia pestis is critical assessment of the evolutionary status of natural populations belonging to formal, taxonomic species of implicated rodents. We present data on chromosomal and hemoglobin variation in sympatric populations and laboratory produced hybrids that give unequivocal evidence for at least two biologicql species in the taxon Praomys (Mastomys) natalensis. We argue for the usefulness of the biological species concept as a basis for any ecological investigation into pathogen biology.