Biological and biochemical characterization of 36 human and 5 animal congolese stocks of Trypanosoma brucei were performed. One human and all the animal stocks showed a quick adaptation to rodent host whereas the other 35 human stocks were characterized by a low virulence degree (Group 1 of T. gambiense). The virulent stocks showed hybridization patterns specific to the gambiense subspecies. Our results confirm the absence of the T. b. brucei subspecies in the Congo and the low prevalence of domestic animals infected with T. b. gambiense (0.5%). Two cycles of human trypanosomiasis may thus occur in Central Africa: a predominant man-to-man cycle with group 1 trypanosomes and a minor cycle involving an animal reservoir.