Twenty-two cases of naturally occurring encephalitozoonosis in squirrel monkeys are reported from breeding colonies of the Delta Regional Primate Research Center, Covington, La. Characteristic foci of granulomatous inflammation and organisms were demonstrated in brains, kidneys, lungs, adrenals, and livers. Vasculitis and perivasculitis were also common lesions in several organs. At least seven cases were congenital while ten others occurred in monkeys less than nine months old. Granulomatous placentitis, previously unreported in any species due to Encephalitozoon cuniculi, was present in one monkey.