Isoenzyme profiles of 67 stocks of Leishmania donovani sensu lato from across the Old World were compared with those of reference strains of L. donovani sensu stricto, L. infantum, L. major, L. tropica and L. aethiopica using starch-gel electrophoresis of 13 enzymes (GPI, GD, ES, PGM, PEPD, NH, ASAT, ALAT, PK, MPI, 6PGD, SOD, MDH). 12 zymodemes were seen. Isolates from man, Canis familiaris, Vulpes vulpes, Rattus rattus, Arvicanthis sp. and Phkbotomus martini were examined. Several zymodemes comprised stocks from man and C. familiaris, two of which also included wild animal isolates. Isolates from cases of post-kala-azar dermal leishmaniasis and visceral leishmaniasis were indistinguishable. L. donovani s.l., including L. donovani s.s. and L. infantum, formed a coherent group with a striking degree of enzymic homogeneity. Only one enzyme pattern was held in common with the L. tropica and L. aethiopica reference strains and two with the L. major reference strain enzyme profile.