The nuclear DNAs of 71 trypanosome stocks from different African countries, representative of the three Trypanosoma brucei subspecies, and one T. evansi stock, have been analysed by the combined use of restriction endonuclease digestion, gel electrophoresis and molecular hybridization with both trypanosome surface-antigen-specific and undefined genomic DNA probes. In contrast with T. brucei brucei and T. brucei rhodesiense stocks, all the T. b. gambiense stocks are characterized by a conserved, specific DNA band pattern, regardless of the probe. This allows T. b. gambiense to be non-ambiguously identified. On the contrary, T. b. brucei and T. b. rhodesiense, which could not be discriminated by the same criteria, both yield highly variable DNA band patterns. Our data confirm that domestic animals like pig, dog and sheep constitute a potential reservoir for T. b. gambiense. Using a numerical analysis of the DNA hybridization patterns we have measured the degree of similarity between the 72 trypanosome stocks. This investigation shows that all T. b. gambiense stocks are included in the same homogeneous population, while the stocks from the two other subspecies seem to be distributed in several heterogeneous groups, some of these showing correlation with the geographical origin of the trypanosomes. It is concluded that (i) T. b. gambiense stands out as a real subspecies that has undergone a distinct evolution relative to the ‘non-gambiense’ group, (ii) the alleged T. b. rhodesiense subspecies does not fit with any of the groups evidenced by our cladistic analysis and hence does not appear as a distinct subspecies and (iii) ‘non-gambiense’ trypanosomes are probably evolving much more rapidly than T. b. gambiense. Different aspects of trypanosome relationships and evolution are discussed.