A study was made on the efficacy of a commercial ultraviolet (UV) sterilizer in reducing the number of bacteria and yeasts in a saline, closed ystem marine mammal complex. UV irradiation was effective in lowering bacterial counts in the effluent of the unit (>75% reduction), but bacteria in more remote parts of the water system reached levels equal to or greater than pre-UV counts. Yeast reduction was considerably less, and a trend similar to that of the bacteria was observed in remote sections of the water system. It is concluded that UV irradiation is of limited value in the disinfection of marine mammal water. Factors contributing to the poor performance of the sterilizer were the long recycle time of the water and lack of a residual effect.