Three different proteinase inhibitors purified from crayfish blood, a 23 kDa inhibitor of subtilisin, a 155 kDa trypsin-inhibitor (pacifastin) and an α2-macroglobulin were tested for their inhibitory activities against extracellular proteinases from the specialized crayfish parasite Aphanomyces astaci, the saprotrophic A. laevis and the plant parasitic A. chochlioides and A. euteiches. All three crayfish inhibitors were effective against extracellular proteinases from the different Aphanomyces species when defined peptides were used as substrates for the proteinases. Proteolytic activities from A. astaci against a complex substrate, casein were reduced by any of the three crayfish proteinase inhibitors tested. It is, therefore, possible that these proteinase inhibitors may reduce the proteolytic breakdown exerted by A. astaci proteinases during an infection.