The production of protease activity by the sugarbeet pathogen Aphanomyces cochlioides, the legume pathogen A. euteiches, and the fish pathogen Saprolegnia parasitica was examined. Protease activity was readily detected in supernatants of water cultures of each organism using autoclaved host tissue as a nutrient source. Most of the protease isozymes extracted from sugarbeet and pea seedlings infected with A. cochlioides and A. euteiches, respectively, co-migrated with enzymes produced by the pathogens in culture. In inoculated sugarbeet seedlings, the protease activities were detected prior to or concommitent with the onset of disease symptoms and the activities were capable of digesting protein extracted from sugarbeet hypocotyls. Use of class-specific inhibitors indicated that a portion of the protease activity was of the trypsin-class. Trypsin-like isozymes that possessed a relatively fast electrophoretic migration were detected in the A. cochlioides, A. euteiches, and S. parasitica protease complements, whereas the remaining isozymes were not affected by any of the inhibitors tested. Proteinaceous trypsin inhibitors from the legumes lima bean (Phaseolus lunatus) and soybean (Glycine max) inhibited the trypsin-like isozymes from A. cochlioides, but not A. euteiches, whereas low molecular weight, synthetic trypsin inhibitors inhibited these isozymes from both pathogen sources. The potential role of protease inhibition in determining host range in phytopathogenic Aphanomyces species is discussed.