The first case of phocine distemper in a seal from Canadian waters and the first case of clinical phocine distemper in a harp seal, Phoca groenlandica, is reported. A two-month-old female harp seal stranded on Prince Edward Island in May 1991. Significant clinical findings were lethargy and severe conjunctivitis. Pulmonary congestion was the main necropsy finding, and histological lesions included diffuse demyelinating nonsuppurative encephalitis and mild multifocal interstitial pneumonia. Acidophilic intracytoplasmic and intranuclear inclusions were present in cerebral neurons and astrocytes. Immunoperoxidase staining confirmed phocine distemper virus (PDV) antigen in the cytoplasm and nuclei of neurons, bronchiolar gland epithelium and transitional epithelium of the bladder. Infectivity titers of canine distemper virus (CDV) (Onderstpoort strain) and a morbillivirus isolated from a grey seal were significantly reduced by serum from the harp seal.