"Ich" or white spot disease is a common condition in freshwater aquarium fishes. It is caused by a ciliated protozoan parasite, Ichthiophthirius multifilis. The young free swimming "tomite" invades the skin where it becomes encysted, causing disease. Treatment and control of "ich" relies on an understanding of the parasite's life cycle, and on the use of simply formulated compounds. This classical condition is presented because it provides a model of the disease process in the gills and skin of fishes, and because it exemplifies the use of biological control.