The pathogenic trypanosomes of cattle in Africa are identified by the traditional parasitological techniques as Trypanosoma brucei, T. evansi, T. equiperdum, T. congolense, T. simiae and T. vivax. The species T. brucei, T. evansi and T. equiperdum on the one hand and T. congolense and T. simiae on the other hand are not distinguishable by their morphology in a thin film. These trypanosomes are distinguished by their characteristic pathogenicity and epidemiology. New tools developed by molecular biologists now make it possible to characterise these parasites both in the vectors and the hosts. The polymerase chain reaction (PCR) makes possible the separation of T. congolense from T. simiae and the characterisation of five different 'taxa' within the species T. congolense. Epidemiological studies undertaken in Burkina Faso, combining the characterisation of the parasites in cattle and tsetse flies, the identification of the origin of the blood meals in the vectors and the precise location of collection, give a more complete image of the transmission of these parasites. These studies are essential for the design of more effective methods of control.