Five cases of acute toxoplasmosis in caged squirrel monkeys (Saimiri sciureus) were reviewed. The early stages of systemic toxoplasmosis were present in all cases. In the liver, there were multiple foci of hepatocellular necrosis. The lung had diffuse interstitial pneumonia. Tachyzoites were mainly seen associated with lesions in the liver, lung, and spleen. Immunohistochemically, tachyzoites reacted with T. gondii antibody. Electron-microscopically, intracellular tachyzoite was surrounded by a thin wall and was in a membrane-bound parasitophorus vacuole.