The inhibitory effects of chitosan on germination and production of zoospore, and growth of vegetative hyphae of Saprolegnia parasitica Coker were examined. Zoospore germination was completely inhibited by chitosan at the concentration of 0.06%. Mycelial growth was not observed on GY media containing 0.05% chitosan. Zoospore production from sporangium was also completely inhibited by 0.05% chitosan. The hyphae affected by chitosan were markedly shrunk and contracted.