A female Atlantic bottle-nosed dolphin (Tursiops truncatus) and her calf were found beached on Picnic Island in Tampa Bay, Florida, USA. Despite therapy the animals died. Necropsy revealed severe pneumonia and lymphadenopathy in the mother and the calf, gastric ulcers and infection with the stomach digenean Braunina cordiformis in the mother, and a large, pale liver in the calf. Toxoplasma gondii was identified by light and electron microscopy and by immunohistochemistry in tissues of both animals. Toxoplasma gondii was associated with interstitial pneumonia, necrotizing adrenalitis, and cardiac myonecrosis in the mother and with lymphoid necrosis in both dolphins. The source of infection and the relationship to the recent dolphin beachings along the eastern seacoast of North America are unknown. This is the first report of toxoplasmosis in cetaceans.