Ten animals from a reputedly trypanosensitive bovine breed, the Zebu, and ten from a reputedly trypanotolerant breed, the Baoulé, were exposed to natural trypanosome challenge in an area of high Glossina density. The ten Zebus and five of the Baoulés died within 6 to 11 weeks with heavy parasitaemia and severe anaemia. The five other Baoulés showed little or no parasitaemia, no anaemia and were all in good condition after weeks of exposure: Seven animals of a herd of Baoulé/Ndama crosses indigenous to the same area were followed concurrently and showed very rare or no patent parasitaemia. Clearly, only some individuals within a "trypanotolerant breed" are able to withstand natural fly challenge. Selection of trypanotolerant animals has to be done on an individual basis and not on the biometrical characteristics of the breed.