A 16-year-old Diana monkey (Cercopithecus diana diana) with chronic renal insufficiency was evaluated for progressive weight loss. The examination revealed a subcutaneous mass on the left carpus. Fluid was aspirated and disclosed numerous, shiny, nonbirefringent particles that stained bright red with bright red S. The mineral present was identified as apatite by high resolution X-ray crystallographic analysis. Later, similar masses developed near the left ischium and the right tarsus. The monkey was euthanatized and a necropsy performed. The histological studies revealed profuse amounts of hematoxyphilic material surrounded by fibrosis, histiocytes, and multinucleated giant cells. Transmission electron microscopy showed clumps of intracellular fine needle-shaped crystals typical for apatite. These apatite deposits were probably related to an increased serum calcium x phosphorus product secondary to the renal disease.