Cowpea lines tested with suspensions of the bacterial pustule pathogen (Xanthomonas sp.), using the infiltration inoculation method, showed three types of reaction - brown hypersensitive resistant (BRH), resistant (R) and susceptible (S). Ten isolates, inoculated in four cowpea lines representing BHR, R and S reactions, could be classified into three distinct pathogenic races by the ability of an isolate to overcome the BHR reaction in one or two lines. The R reaction was table against all the three races. Race 1 may be widely prevalent in West Africa and races 2 and 3 in East Africa. A set of cowpea cvs with differential reactions to identify these races is proposed. In the screening of 234 cowpea lines, 166 reacted uniformly BHR to race 1 and S to races 2 and 3, 23 were BHR to races 1 and 2 but S to race 3, and 11 were R to all three races. The advantages of infiltration over the spray method of inoculation for screening cowpea lines against bacterial pustule are discussed.