A newborn diprosopic female calf had a partially duplicated head with two faces each exhibiting a mouth, a snout, an anomalous incomplete mandible, two eyes and a lateral ear. A single ear with two small auditory canals was present on the midline between the two medial eyes. A type 1 persistent truncus arteriosus and hypoplasia of the thoracic portion of thymus were the most outstanding extracranial defects. In the heart, a persistent foramen secundum and a large patent foramen ovale allowed communication between the right and left atria. In the right ventricle, the small conus arteriosus was separated in part from the inflow tract by an anomalous 'septomarginal muscular septum'. An interventricular septal defect was also present. A large undivided truncus arteriosus, exhibiting a tricuspid truncal valve at its origin, arose for the most part from the conus arteriosus of the right ventricle. The truncus gave rise to the brachiocephalic trunk, the aortic arch, a small pulmonary trunk, from which the left and right pulmonary arteries emerged, and two coronary arteries. The etiology and pathogenetic mechanisms implicated in the appearance of persistent truncus arteriosus are reviewed. It is suggested that a deficit or insufficiency in the cranial neural crest may play a role in the pathogenetic mechanisms leading to the production of persistent truncus arteriosus and related defects in cephalic duplications.