Abstract Eight of 20 newly hatched double-crested cormorants (Phalacrocorax auritus), captured at Doré Lake (Saskatchewan, Canada) and raised in captivity, developed malformed bills when they were 2 to 3 weeks old. Malformation was characterized by abnormal flexure and rotation of the maxilla and mandible, resulting in a crossed bill. By radiography, the premaxillary and dental bones were misshapen. Morphologically similar malformed bills in free-living cormorants have been attributed to exposure to polyhalogenated aromatic hydrocarbons. However, the concentrations of total PCBs in the livers of these captive cormorants with malformed bills and in their diet were lower than have been previously associated with such malformations and were considered too low to have been the cause. The bill malformations may have been caused by deficiency of vitamin D3, because the cormorants were kept indoors without exposure to ultraviolet light and were fed frozen fish that may have been deficient in this vitamin.