An outbreak of possible acute poisoning by Sinapis arvensis occurred in a flock of 50 fat-tailed sheep located in the Semnan province of Iran. Sinapis arvensis is an annual or winter annual plant of the genus Sinapis in the family Brassicaceae, commonly known as field mustard, wild mustard or charlock. The poisonous constituents are volatile oil of mustard, the alkaloid sinapin, and the alkaloidal glucoside sinalbin. The flock was grazing in land containing high amounts of wild mustard (Sinapis arvensis) in late spring. Seven sheep (aged between 1 and 5 years) died within approximately 3 days. The affected animals displayed signs of depression, reluctance to move, tachycardia, tachypnea, mucoid and hemorrhagic nasal discharges, pale conjunctiva, ataxia, abdominal pain, bruxism, and anorexia. Rectal temperature in these animals was normal to high (39-41.5 °C). Ruminal movements were reduced (1-2/min). Serum biochemical levels in affected sheep showed marked increases of blood urea nitrogen (BUN), Gamma glutamyl transferase (GGT), magnesium (Mg), and phosphorous (P) and a marked decrease in calcium (Ca). In urinalysis, marked hemoglobinuria and proteinuria were observed. Necropsy findings included congestion in lungs and hemorrhage on the epicardial and endocardial heart surfaces, on the surface and medulla of the kidneys, and abomasal mucosa. The liver was also congested with a nutmeg pattern. Rumen contents included digested materials and large quantities of seeds and stems of wild mustard. For the first time, our findings confirmed wild mustard toxicosis in sheep in Iran.