Solanum balbocastanumDun. was collected extensively throughout its range in Mexico and Guatemala. Experimental self- and cross-pollinations showed that the species is highly self-sterile and consists of cross-sterile and cross-fertile individuals. Self-sterility may be due to the presence of incompatibility factors or to triploidy. Authentic hybrids were produced betweenS. bulbocastanumandS. trifidumCorrell, and betweenS. bulbocastanumandS. pinnatisectumDun.After inoculation with race 1.2.3.4 ofPhytophthora infestansthree types of reaction were observed among 1148 seedlings ofS. bulbocastanum: immunity with no perceptible lesions, resistance expressed by non-sporulating lesions of the hypersensitive type, and susceptibility indicated by large sporulating necrotic lesions. Seedlings resistant to an isolate of race 1.2.3.4 of Canadian origin did not always show the same level of resistance to a Mexican isolate of the same race. Varying degrees of field resistance were observed among seedling plants, while tuber-propagated plants were generally field immune.Solanum bulbocastanumis considered a mixture of resistant and susceptible genotypes.