a serious disease of young freshwater fish, resulting in mass deaths. It occurs everywhere and is caused by a flagellated protozoan ( Costia necatrix). Infected fry are identified by the formation of dull, bluish gray spots that in severe infestations coalesce into a solid coating. Often the fins decompose and the fin rays become exposed. Salt, formaldehyde, and other baths are used to treat the disease. Measures for the prevention of costiasis include feeding the fry good-quality live food, improving hygienic conditions in ponds, and exterminating the parasites in the environment.