Despite the increasing use of the squirrel monkey in nutritional studies, little is known of the dietary requirements of this species. Twelve young (700 gm initial wt) male squirrel monkeys were allotted to four groups of three animals each, and were offered ad libitum, during 24 weeks, isocaloric diets in which the amounts of protein were 25%, 12.5%, 9%, or 6%. The dietary protein employed was a purified soya protein and was supplemented with 2.1 gm% of methionine to obtain the same content of this amino acid as present in casein. The diets contained amounts of vitamins and essential food factors in excess of the estimated normal requirements. The results indicate that the diets with 25% protein permit the growth and the normal maintenance of several functional and morphologic parameters studied. An almost similar effect was observed with 12.5% protein diets. The protein intake in monkeys fed this diet was 8 gm/day/kg of body weight, and it appears to be the minimal normal protein requirement for this species in captivity under the experimental conditions employed in our work. The effect of 9% and 6% protein diets was manifested by poor ponderal growth, decrease in albumin/globulin ratio, decrease in hemoglobin concentration, decrease in BUN, and increase in serum alkaline-phosphatase activity. In addition, the livers of animals on the low-protein diets had moderate to severe ultrastructural alterations.