<p>The manuscript reports accidental water buffalo (<i>Bubalus bubalis</i>) poisoning by monensin, a food supplement commonly used to feed cattle, and indicate some relevant aspects related to buffalo breeding. Most of the buffalo intoxication reports are accidental and occur in areas where buffalo breeding is not a traditional activity or in new buffalo breed without expert assistance. The present communication reports accidental buffalo poisoning due to monensin consumption at one corral that usually breed bovine to slaughter, a situation not reported before in Brazil. Monensin poisoning hypothesis was confirmed through investigation conducted by veterinarians using epidemiology, clinical signs and specific laboratory tests on food, blood and serum and biological samples obtained during a sick buffalo necropsy. Intoxicated animals presented apathy, anorexia, diarrhea, muscular weakness, locomotion disorders, dyspnea, tachycardia, jugular distension, pulse and recumbency, suggesting acute intoxication in the animals. Complementary diagnoses included myocarditis, moderate multifocal myonecrosis, myositis with lymphocytic infiltration and moderate multifocal oedema, presence of inflammatory biomarkers and normocytic normochromic anemia, typical of muscle damage observed in monensin poisoning.</p>