A fetus suffering from tachycardia of more than 200 beats per minute was found to have an atrial flutter with two-to-one atrioventricular conductional block by the fetal ultrasonogram at 33 weeks of gestation. The tachycardia was successfully converted to sinus rhythm with maternal digitalization. The heart rate stayed in a normal sinus rhythm after the baby was born, however, an incomplete form of cor triatriatum was noted on the echocardiogram examination.