Throughout this century, considerable confusion has arisen over the specific status of members of the subgenus Nannomonas. At the present time, only two species are generally recognized, T.(N.) congolense and T.(N.) simiae. However, many differences still exist in morphological and behavioural characteristics of T.congolense stocks, which led in the past, to numerous designations of species, subspecies and varieties. In an attempt to clarify the problems, 78 stocks of T.congolense were characterized intrinsically by enzyme polymorphism. Conditions devised for thin-layer starch-gel electrophoresis of the isoenzymes of trypanosomes from other subgenera were modified for 14 enzymes: L-threonine 3-dehydrogenase (TDH), purine nucleoside hydrolase (NH), pyruvate kinase (PK), glyceraldehyde phosphate dehydrogenase (GAPDH), aspartate aminotransferase (ASAT), phoepho-glucomutase (PGM), alanine aminotransferase (ALAT), malate dehydro-genase (decarboxylating NADP) (ME), malate dehydrogenase (MDH), glucose phosphate isomerase (GPI) and four peptidases using either L-leucylglycylglycine (PEP 1) or L-leucyl-L-alanine (PEP 2) as substrate. Eight of these enzymes readily distinguished T.congolense from representatives of the subgenera Trypanozoon and Duttonella. The numbers of different isoenzyme patterns for T. congolense, each often with multiple banding, ranged from one for TDH, PK and NH, to ten for GPI. Altogether 71 different isoenzyme bands were seen and from the various combinations of these bands, 75 zymodemes were found among the 78 atocke of T. congolense examined. The degree of relationship between the zymodemes was established by computer analysis. A major dichotomy existed between stocks originating from savannah regions and those from riverine or forest areas. Thia distinction was most obvious in the GPI, PEP 1 and PEP 2 isoenzyme patterns. Besides morphological comparisons of these two major zymodemes, initial investigations of behavioural characteristics included transmissibility by Glossina app., drug sensitivity, infectivity and susceptibility to disease in different strains of inbred mice. Differences between the two majpr zymodemes were found in morphology and drug sensitivity; the savannah zymodeme appeared to be more efficiently transmitted than the riverine/forest zymodeme by the savannah tests, fly, G.morsitans. The susceptibility to disease of two strains of inbred mice varied with different trypanosome stocks; there was no correlation with the two major zymodemes. The relevance of these findings is disussed.