Using an improved method for evaluating powdery mildew (Sphaerotheca cucurbitae) resistance in cucumber, experiments were conducted for the breeding of varieties resistant to powdery mildew that would be adapted to greenhouse cultivation during the period from winter to spring in Japan. The relationship between the resistance to the pathogen and the ambient temperature was analyzed by spray inoculation of a conidial suspension under controlled conditions. Although the varieties, Asomidori-5-gou and Natsufushinari were resistant at temperatures between 25 to 30℃, they became susceptible at 15 to 20℃. The resistance was affected by the duration of the exposure to 30 and 15℃ during the day. To screen resistant varieties and lines independently of the temperature, 295 cucumber accessions were inoculated at 20 and 26℃. All the accessions tested could be divided into 3 types based on the difference in the resistance response to temperature. Many of the resistant accessions were Chinese varieties and lines. PI197088-5, a progeny of PI197088 which originated in India, displayed the highest level of resistance among all the accessions. Thus PI197088-5, whose resistance is temperature-independent appears to be the most suitable parent for the breeding of powdery mildew-resistant cucumber varieties. The inheritance of powdery mildew resistance in PI197088-5 was analyzed using Natsufushinari (resistant under high temperature conditions) and Sharp 1 (susceptible). As a result it was considered that the powdery mildew resistance in PI197088-5 was due to 2 gene pairs, a recessive gene and an incompletely dominant gene, with the genotype designated as aaBB, while the genotype of Natsufushinari was designated as aabb and that of Sharp 1 as AAbb. The nature of the host-pathogen interaction in PI197088-1 which is the sib-line of PI197088-5 was observed microscopically. The retardation of hyphal growth and development was observed at 2 days after inoculation. Conidia were hardly formed on the epidermis and the number of haustoria in the epidermal cells of PI197088-1 was much lower than that in the susceptible variety Sharp 1. Fluorescence of epidermal cells was observed along the hyphae in PI197088-1 at 6 days after the inoculation, suggesting the existence of a hypersensitive reaction.