Three resistant accessions: L. hirsutum LA1775, L. pennellii LA716, L. chilense LA2747, and the susceptible breeding line A100, were crossed to develop F1, F2 and BC1 populations for genetic analysis of tomato resistance to Oidium lycopersicum Cooke & Massee, emend. Noordeloos & Loerakker, the causal agent of powdery mildew. The resistances in all the studied wild species of Lycopersicon were dominant, but controlled by different numbers of genes depending on the source of resistance. Two incompletely dominant genes control the resistance in L. hirsutum, but one major gene has a stronger expression than the second minor gene. L. pennellii carries three cumulative dominant genes. The resistance of L. chilense is governed by one partially dominant gene that is less effective than the resistance genes of L. hirsutum.