Rickets and osteomalacia have been reported frequently in captive callitrichids. Some have assumed that these conditions are a consequence of unmet, unusually high requirements for vitamin D and that these high requirements are characteristic of all New World primates. As a consequence, certain commercial diets formulated for New World primates contain such high concentrations of vitamin D that their consumption by other species has resulted in signs of vitamin D toxicity. This study was conducted to assess the vitamin D status of captive cotton-top tamarins consuming diets providing either 2,500 or 26,000 IU of vitamin D 3 /kg dry matter. These diets had been consumed for at least 2 years before the study, with the lower vitamin D intakes by six tamarins (0.5 to 9 years old) in a zoo colony and the higher vitamin D intakes by 24 tamarins (2 to 12 years old) in a pharmaceutical research laboratory. Although not measured in this study, none of the dietary ingredients has been shown to contain vitamin D 2 . Serum 25hydroxyvitamin D (25(OH)D) concentrations in the captive tamarins were compared with serum 25(OH)D concentrations (range, 25.5-120 ng/mL; 64-300 nmol/ L) reported by others in healthy wild tamarins in Colombia, South America. Concentrations of 25(OH)D in serum from zoo tamarins consuming 2,500 IU vitamin D 3 /kg dietary dry matter ranged from 48 to 236 ng/mL (120-590 nmol/ L), whereas those in serum from laboratory tamarins fed 26,000 IU vitamin D 3 / kg dietary dry matter ranged from 11 to 560 ng/mL (28-1,400 nmol/L), with no significant (P > 0.05) association between serum 25(OH)D concentration and sex or age. However, in the laboratory tamarins, serum 25(OH)D concentrations