An 11-year-old multiparous Thoroughbred mare with a history of colic was referred for further evaluation. Clinical findings at the time of initial examination included arrhythmia, tachycardia, tachypnoea and hypothermia without marked signs of abdominal pain.Electrocardiography revealed ventricular tachycardia at a rate of 211 beats/min. Lidocaine was administered as a constant rate infusion to treat the arrhythmia, and the heart rate and rhythm were normalised. There were no abnormal findings on ultrasonography of the abdomen and thorax. An echocardiographic examination revealed a hypoechoic spherical area within the muscular part of the intraventricular septum. Unfortunately, the mare died suddenly 3 days after treatment. Post-mortem examination confirmed the presence of an encapsulated abscess in the intraventricular septum. Staphylococcus aureus and Staphylococcus chromogenes were isolated from the lesion. It was thought to be responsible for the arrhythmia seen in this case. This is the first case of a septal myocardial abscess in a horse.