One hundred eighty sexually mature Saguinus mystax were imported from Peru in six lots over a period of 1 year. Within 1 year after arrival, the mortality was 60% and the majority of the tamarins showed signs similar to "wasting marmoset syndrome" (WMS). In an effort to improve the survival rate, an open formula diet replaced the commercial closed formula diet that had been fed since arrival of the tamarins. The open formula diet contained 26.2% crude protein, 12.3% ether extract, 43.3% nitrogen free extract and 5.9% crude fiber on a dry matter basis. The diet was evaluated on the basis of palatability, weight gain, mortality, digestibility, nitrogen balance, serum biochemical parameters and blood counts. The mean daily consumption on an as-is basis was 44.8g or 335 Kcal gross energy/Kg of body wt./day. During the 3 month open formula diet evaluation period average weight increased by 56g (p less than .05), mortality decreased demonstratively, and alopecia and chronic diarrhea were nearly eliminated. Mean daily gross energy intake for S. mystax (335 Kcal/Kg of body wt/day) was substantially greater than previously reported values for callitrichids. WMS signs observed in the S. mystax colony were controlled by providing what appears to be an adequate diet.