Abstract: A Thoroughbred horse with clinical signs of inappetence, loss of body weight, poor performance and scrotal sac infection following castrationwas examined. Because of irregularity in heart rhythm, ECG was recorded and on the base of normal rhythm criteria, atrial fibrillation was diagnosed. This is believed to be the first reported case of atrial fibrillation in horses in Iran. J.Vet.Res. 62,2:53-55,2007.