A study has been made of powdery mildew (Erysiphe graminis DC.) in Canada, including its distribution, the physiologic specialization of its two varieties, Hordei and Tritici, and the reactions of barley and wheat hosts to the prevalent races of the two varieties of mildew. The fungus occurs throughout Canada but it is most prevalent in British Columbia, Ontario, and Quebec. Nine races of E. graminis Hordei and three of E. graminis Tritici have been identified. Races 3, 6, 8, and 9 of the former, and races 4 and 5 of the latter are the most common in Canada. At a higher temperature (25 °C.) practically all the races of mildew developed a more resistant type of infection than at a lower temperature (15 °C.). The only exception was race 6 of E. graminis Hordei, which thrived fairly well at the higher temperature.The reactions of 246 varieties and strains of barley to nine races of barley mildew and the reactions of 124 wheat varieties and strains to two races of wheat mildew indicate that there are varieties of both cereals that are resistant to powdery mildew.