Ichthyophthirius multifiliis (Ich) was added to four aquaria in a 36-tank fish culture recirculation system to evaluate the effectiveness of ultraviolet (UV) light preventing spread of infectious agents. No spread of disease was evident during trials while a 250,000 µW sec cm^-2 UV light source was operating. However, Ich spread without the UV light. Total aerobic bacteria were decreased 100 to 1000-fold in water sampled immediately past the UV source before being returned to the tanks, but numbers were not affected in the tanks. The water quality parameters measured were not changed by UV treatment. UV light appears to be a useful method for containing an infectious agent within a contaminated tank in a recirculating system.