The inhibition of Kudoa septempunctata by green tea extract, black tea extract, and coffee extract were studied. Incubation of about 104 Kudoa spores with green tea extract, black tea extract, or coffee extract at 25℃ for 4 hr reduced the survival ratio of Kudoa to 0%. While coffee extract and green tea extract contain approximately 2 and 1 mM of caffeine, respectively, the incubation of Kudoa spores with 2 and 1 mM of caffeine reduced its survival ratio to 68.2 and 93.3%, respectively. Although green tea extract and black tea extract contain over 1 mM of catechin, incubation with 0.01 mM of catechin was enough to reduce the survival ratio of Kudoa to 20%. These results suggested that green tea extract, black tea extract, and coffee extract have strong inhibitory effects on Kudoa and the effects of green tea extract and black tea extract are mainly manifested through catechin.