A 2.5-y-old, male common marmoset (Callithrix jacchus jacchus) developed a 2-cm, interscapular, subcutaneous mass with variably firm and cystic areas. Radiographs demonstrated a radiodense mass in close proximity to a previously implanted microchip. Fine-needle aspiration yielded a chalky liquid that, on cytologic examination, contained amorphous debris. Aerobic and anaerobic cultures were negative. Surgical excision required extensive dissection, with the mass infiltrating deep to the scapula and extending to the mammary gland. The mass weighed 30 g and comprised 10% of the animal's body weight. Microscopic examination demonstrated multifocal, variably sized, amorphous aggregates of granular, basophilic material (mineral) in the subcutis and extending to skeletal muscle. Mineral deposits were surrounded by macrophages, giant cells, and fibrous connective tissue. A focal area of ectopic bone production was present. Crystallographic analysis and x-ray diffractometry determined the material to be comprised of 100% hydroxyapatite. These findings were consistent with a diagnosis of calcinosis circumscripta. Systemic metabolic abnormalities were excluded based on examination of complete blood count, serum chemistry, and ionized calcium. Calcinosis circumscripta in the common marmoset has not previously been reported, although the lesion has been reported to occur in rhesus macaques and is well described in man and dogs. Accumulation of calcium deposits and production of ectopic bone in a marmoset is interesting in light of this species's unique calcium and vitamin D metabolism.