1. The catalase activity was determined of non-infected wheat leaves and of those infected with stem rust, Puccinia graminis tritici. Wheat varieties and rust races were selected so that both susceptible and resistant reactions could be studied. 2. In an infected susceptible host the catalase activity of the first seedling leaf is slightly higher than that in non-infected leaves during the early stages of infection. Activity in the infected susceptible leaves may drop slightly below that of the non-infected leaves at the time rust flecks appear. Once urediospore production begins, the activity of the infected leaves becomes greater than that of the non-infected leaves and continues to rise until rust sporulation is abundant. The increased activity following sporulation appears to be due to the rust urediospores. 3. During the early phases of stem-rust infection in a resistant host there is a marked increase in catalase activity in the infected leaves as compared with the non-infected leaves. As the rust continues to develop, catalase activity of infected leaves drops back toward that of non-infected leaves.