A study has been made of various aspects of the biology of Erysiphe graminis DC., including its distribution and prevalence in Canada, method of overwintering, physiologic specialization, and the effects of various environmental factors on the development of different stages of the organism. The fungus causes the powdery mildew of cereals and certain grasses. It occurs in every province of Canada, but is more prevalent in British Columbia and the five eastern provinces. The evidence indicates that the fungus overwinters as mycelial mats on dead straw and as mycelial infections on overwintering hosts. Three new physiologic races of barley mildew have been identified, making a total of seven races isolated in Canada. In wheat mildew only Race 1 was isolated. A study of the stability of six races of barley mildew showed that they are distinct biologic entities comparable to physiologic races in cereal rusts. No evidence was obtained by cross inoculations that any variety of E. graminis, except possibly E. graminis Agropyri, can attack a non-suscept, or an immune variety of its host, or that injury of such hosts renders them susceptible. Conidia of E. graminis germinate on immune hosts and penetration proceeds as far as the papilla stage. Physiologic resistance manifests itself in some hosts by the death of the infected cells and in others by the distortion of the haustoria. The evidence indicates that the organism is homothallic. The cardinal temperatures for the germination of conidia are 0°, 10°, and 35 °C., while the best development of mildew occurs between 15° and 20 °C. Alternating temperatures induce perithecial development. Light is not an important factor, except indirectly, in the development of E. graminis. Alternate drying and wetting of perithecia is necessary to induce the formation of ascospores, but other stages are favoured by relatively dry conditions. Conidia germinate well even at zero humidity. Sprinkling infected seedlings with water checked the development of the disease. The addition to the soil of certain fertilizers and other chemicals commonly claimed to increase host resistance to powdery mildew did not increase the resistance of susceptible seedlings of wheat and barley.