An enzootic of toxoplasmosis occurred in caged squirrel monkeys (Saimiri sciureus, a New World primate) which resulted in 9 deaths during an interval of 22 months. Diagnosis was based on morphology of the organism, character and distribution of the lesions, and laboratory history of the monkeys. The character of the lesions was essentially necrotic. The chronological incidence of the disease and distribution of lesions and organisms are tabulated. Trypanosoma cruzi which causes Chaga's disease is differentiated morphologically from toxoplasma. Incidence and lesions of the enzootic suggest 5 factors to be considered in the pathogenesis of toxoplasmosis in squirrel monkeys. They are: (1) lack of protective immunity, (2) local concentration of toxin and/or catabolites of reproduction of the organism, (3) individual tissue susceptibility to the organism, (4) capillary thrombosis, and (5) ability of the individual to adapt to its environment.
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