Disseminated toxoplasmosis was diagnosed in a female California sea lion that had been used in thermoregulation studies for about 4 years while housed in a small enclosure with a fresh-water pool at the University of Hawaii, Honolulu. Toxoplasma gondii was demonstrated in and around necrotic lesions of the heart and the stomach. Source of the infection was not established; however, studies on toxoplasmosis in cats were being conducted at the institution, and common caretakers attended both groups of animals.