Glossinainfected with African trypanosomes infest 107Km2of intertropical Africa. Ten thousand cases of human sleeping sickness are officially recorded each year and 35 times 106human beings are at risk. Animal trypanosomiasis impedes the use of 7times 106Km2of land adequate for cattle raising and constitutes a major constraint to increasing protein production in Africa.Several approaches are used to combat trypanosomiasis (a) vector eradication has been successful in certain denned situations but cannot be realistically extended to the whole area at risk, (b) prophylactic drugs, which are too toxic for humans, are widely used to protect cattle. This practice has led to field resistance to many drugs used in the past (reviewed in Holmes&Scott 1982) and the appearance of resistance to the last available prophylactic drug, isometamidium, is being reported (Bourn&Scott 1978, Kiipper&Wolters 1983, Pinder&Authie 1984), (c) the hope for a vaccine has been largely abandoned, in the present state of knowledge, due to the considerable extent of antigenic diversity in trypanosomes (reviewed in Doyle 1977, Turner 1982, Roelants&Pinder 1984), (d) consequently, the possible use of certain West African breeds of cattle, which appear resistant to trypanosomiasis, has been emphasized as a solution to this problem in domestic animals. The analysis of this natural resistance is the subject of the present essay.