Myxosarcoma is an unusual neoplasm in the dog, most seen in the skin, and seldom in the heart and in the liver. The authors describe a case of a 5-years old mongrel dog showing abdominal distension, weakness, and decreased appetite. An exploratory celiotomy showed a massive non-encapsulated nodular tumour, adhered on the mesentery, measuring 30cm in diameter. Histopatology identified the tumour as a myxosarcoma. There is not previous record of mesenteric myxosarcoma.