This study compared the antifungal activities of bronopol and 2-methyl-4-isothiazolin-3-one (MT) against members of the genus Saprolegnia in the family saprolegniaceae, which have the ability to infect fish. The minimum inhibitory concentration of bronopol was 100-200 mg/l but for MT it was 31 mg/l. Concerning fungicidal effects against the hyphae, treatment with bronopol at 100-200 mg/l for 30 min or treatment with MT at 25-50 mg/l for 30 min was effective in killing the vegetative stage of all fungal strains tested. Treatment with bronopol at 100 mg/l for 60 min or treatment with MT at 25-50 mg/l for 60 min was effective in killing the zoospores of all fungal strains tested. These results showed that MT was a more effective antifungal agent than bronopol against infectious members of the genus Saprolegnia.