The acid and alkaline invertases of mature leaves of Pisum sativum were partially purified and characterized prior to determining the effects of powdery mildew (Erysiphe pisi) infection on their activities. Seven days after inoculation, infection resulted in significant increases in the activities of both acid and alkaline invertases in whole leaves of cv. Onward, but a significant increase in the activity of only alkaline invertase in isolated mesophyll of cv. Argenteum. Activity of α-glucosidase was determined in leaves and it was concluded that this enzyme could not be responsible for the increased sucrose hydrolase activity observed in host tissues. Seven days after inoculation, infection caused a significant decrease in leaf starch content, a decrease in the activity of ADP-glucose pyrophosphorylase, and no significant change in the activity of α-glucan phosphorylase or content of inorganic phosphate. It was proposed that the decreased content of starch is a result of decreased starch synthesis.