a genus of protozoa of the order Holotricha comprising the one species {Ichthyophthirius multifil-iis). It parasitizes fish that live in fresh and slightly saline waters, under the epithelium of the skin and gills, under the cornea of the eye, and often the fins; it produces whitish swellings of the skin—pustules. At 28°C the growth of Ichthyophthirius in the fish lasts about one day and at 20°C, seven days. Mature infusori-ans (up to 1 mm in size)—trophonts—emerge from the pustules into the water and, upon falling to the bottom of the water, become encysted. In the cyst, division occurs eight to 12 times and 500 to 1,000 (sometimes up to 2, 000) tiny forms (20–microns in diameter) develop, which are covered with cilia. Known as tomites, these forms emerge from the cyst into thewater and infect other fish. Ichthyophthirius often cause massdestruction of fish, particularly of fry. Their control is effectedwith chemical agents.