This paper describes clinical and pathomorphological features of myasthenia gravis in a 7-year-old male German Shepherd dog. At clinical examination, many neurological deficits as well as swallowing and urinating problems were detected. Radiological examination revealed megaoesophagus and the ELISA test showed an elevated serum acetylcholine receptor (AChR) antibody titer. No improvement was observed after the administration of edrophonium hydrochloride. The diagnosis of myasthenia gravis was confirmed by electromyography. Despite the administration of pyridostygmine bromide, dexamethasone and antibiotics, no improvement followed, and the dog died. Necropsy was conducted: histopathological changes in the esophagus, cranial tibial muscle and lungs were described as muscular atrophy, muscular fibrosis and aspiration pneumonia. The dog’s littermate was examined and showed an elevated AChR antibody titer too, but was asymptomatic.