A 1-year-old male neutered Australian Shepherd dog was referred to a veterinary specialist for evaluation of chronic lameness, contractures, and exercise intolerance. Serum creatine kinase was elevated, and electromyography showed complex repetitive discharges. Cardiomyopathy was clinically identified. Muscle biopsies evaluated by light and electron microscopy, and immunohistochemistry revealed the presence of a myofibrillar myopathy with accumulation of desmin and other proteins. These observations represent the first report of myofibrillar myopathy in the dog.