Rainbow trout fry developed whirling disease upon exposure to water contaminated with Myxosoma cerebralis. When similarly contaminated water was irradiated with 2537 Angstrom units of ultraviolet light at dosages of 35,000, 43,000, and 112,000 microwatt sec/cm2, infection of fry was prevented. The minimum effective dosage was not determined in the present experiments. High mortality and a 61% growth inhibition of survivors occurred in the infected fry.