A homogeneous endopolygalacturonase preparation was obtained from a culture filtrate of Gaeumannomyces graminis var. tritici by isoelectrofocusing and carboxymethylcellulose chromatography. The enzyme has a molecular weight of 42kDa an isoelectric point at pH 8·1, and a pH optimum at 4·5-5·5. Different fungal isolates secreted similar amounts of endopolygalacturonase. The same enzyme was present in diseased wheat roots. Endopolygalacturonase activity was detected in inoculated wheat roots 2 days prior to appearance of symptoms. Degradation of pectic substances during infection was revealed in root sections stained with ruthenium red. The possible function of the endopolygalacturonase in pathogenesis is discussed.