Development of the powdery mildew fungus Sphaerotheca fusca on leaves of susceptible, resistant and temperature-sensitive resistant melon cultivars at 21 °C and 26 ° was examined by light and scanning electron microscopy. On the susceptible cv. ‘Rochet’ the time course of the infection process was similar at each temperature. Development of powdery mildew colonies paralleled the progressive branching of mycelia and conidiophore formation. On the resistant cv. PMR-6, however, development was arrested at the primary appressorium stage irrespective of the temperature. When the temperature-sensitive resistant line ANC-57 was infected by a race 1 isolate, incubation temperature had a clear effect on the outcome of the interaction. At 21 °, it took longer for symptoms to appear on cv. ANC-57 than on cv. ‘Rochet’, while colonies were limited and sparse. At 26 ° resistance was complete, there being no development.