<p>This study reports an outbreak of subacute necrotic myopathy in buffaloes associated with the ingestion of a commercial feed formulated for cattle containing monensin. Twenty-nine buffaloes were fed a monensin containing ration, and after two weeks, nine animals developed apathy, anorexia, dark-red urine, recumbency, and ventral abdominal distention. Four of them died and one was euthanized <i>in extremis</i> within two to five days after the onset of clinical signs. Three buffaloes were necropsied, and the main lesions included focally extensive areas of pallor, mainly in the musculature of the thoracic and pelvic limbs. Upon histological examination, these areas were characterized by polyphasic myonecrosis, with floccular and hyaline necrosis of myocytes associated with marked inflammatory infiltration of lymphocytes, plasma cells, and macrophages, as well as areas with proliferation of fibrous connective tissue, proliferation of satellite cells, and occasional mineralization of necrotic fibers. Similar areas of myonecrosis were observed in the heart in the three cases. The history of ingestion of feed containing monensin formulated for cattle and the absence of alternative causes for myonecrosis supported the diagnosis of ionophore poisoning in buffaloes in this report. The subacute pattern of myonecrosis observed in this outbreak is uncommonly reported in buffaloes and it was associated with the prolonged period of ingestion (presumably below toxic levels) of the monensin-containing feed.</p>