The ciliate Ichthyophthirius multifiliis is one of the most pathogenic parasites of fish maintained in captivity. In this study, effects of bacterial extracellular products of Streptomyces griseus SDX-4 against I. multifiliis were determined. The fermentation liquor of S. griseus was extracted successively in a separating funnel with petroleum ether, ethyl acetate, and n-butanol. In vitro assays revealed that the n-butanol extracts (NBu-E) and ethyl acetate extracts (Eto-E) of S. griseus were observed to be more effective against theronts than the other extracts with median effective concentration (EC50) values of 0.86 and 12.5 mg L(-1), respectively, and significantly reduced the survival of the tomonts and the total number of theronts released by the tomonts (P<0.05). All encysted tomonts were killed when the concentration of NBu-E was 30.0 mg L(-1). Results of in vivo test demonstrated that the number of I. multifiliis trophonts on the grass carp treated with NBu-E was markedly lower compared to the control group at 11 days after exposed to theronts (P<0.05). In the control group, 100% mortality was observed owing to heavy I. multifiliis infection at 11 days after the exposure. On the other hand, only 9.5% mortality owing to parasite infection was recorded in the groups treated with the NBu-E (30 mg L(-1)). The median lethal dose (LD50) of NBu-E for grass carp was 152.4 mg L(-1). Our results indicate that n-butanol extract of S. griseus will be useful in aquaculture for controlling I. multifiliis infections.