Truncus arteriosus (TA) was diagnosed in a 6-year-old neutered female domestic short-haired cat by two-dimensional and M-mode echocardiography, colour flow imaging and spectral Doppler examinations. The lesion was characterized by a single large artery originating from the right ventricle. A single ascending aorta and a single pulmonary trunk arose from the common arterial trunk. The residual pulmonary trunk immediately split into left and right branches. The lesion was identified as a type I (TA). This case is of interest because it is the first reported echo-Doppler description of such a malformation in felines, and because of the age of the cat at the time of diagnosis.