The cultivated tomato ( Lycopersicon esculentum) is susceptible to powdery mildew ( Oidium lycopersicum). Six accessions of three related Lycopersicon species show high levels of resistance (Lindhout et al., 1994b). The present research aimed at describing the development of O. lycopersicum on susceptible cv Moneymaker and characterizing the defence response to O. lycopersicum in Lycopersicon accessions by histological analysis. Spore germination and (primary) haustorium formation in resistant accessions were as frequent as in the susceptible L. esculentum cv Moneymaker. A high frequency of necrosis of epidermal cells in which a haustorium was formed appeared to be the major defence response, indicating that resistance to O. lycopersicum in the Lycopersicon genus was predominantly based on the hypersensitive reaction. However, the resistance in L. parviflorum was less associated with hypersensitivity than in other resistant accessions, suggesting the existence of a different but still unknown resistance mechanism. In addition, evidence is provided that the level of resistance could depend on the genetic background and the plant age.