Persistent truncus arteriosus (PTA) was diagnosed in an 8-year-old neutered male Poodle referred for echocardiographic examination prior to anesthesia for surgical correction of bilateral cataract. A single large artery limited by a bicuspid valve and overriding both ventricles was observed with 2 distinct pulmonary arteries arising from the common arterial trunk. A large size interventricular septal defect was associated with a low velocity bidirectional shunt. The lesion was identified as a Type 3 PTA according to Collett and Edwards' classification. Although no clinical signs were reported, the dog presented polycythemia (packed cell volume = 68%) at the time of diagnosis. To our knowledge, this is the first report of an echocardiographic diagnosis of PTA in the dog. Until now, the ante-mortem diagnosis of this congenital heart disease has only been described in the cat. This case is also of interest because of the age of the animal and the total absence of cardio-respiratory signs at the time of diagnosis.