Members of the genus Trichodina with about 60 species described from marine fish and related peritrichous ciliates are the most common parasitic protozoans which are especially harmful to young fish. The species parasitize marine fish by attaching themselves mainly to the gills. Juveniles of seabass observed to be heavily infected with this parasite caused high mortality of over 50 percent of the stock. Trichodina also causes problems to crowded fish reared in cages