A patient with W-P-W syndrome (type A) and disabling supraventricular tachycardia was studied with epicardial mapping prior to surgical interruption of the circus pathway. Analysis of the delta vector and results of the epicardial mapping strongly suggested aberrant pathway located very posteriorly and crossing the atrioventricular sulcus in or adjacent to the interventricular septum. A circus movement responsible for the supraventricular tachycardia was felt to be a mechanism of antegrade A-V conduction and retrograde accessory bundle conduction. Attempts to interrupt the aberrant pathway primarily were unsuccessful, and surgical A-V block was performed. An epicardial demand pacemaker was inserted as a safety feature. Since the operation the patient has been free of supraventricular tachycardia. The postoperative electrocardiograms demonstrated antegrade accessory pathway conduction, but neither antegrade nor retrograde A-V conduction. Failure to interrupt the accessory pathway after incision of the entire right posterior A-V sulcus supported a location with, or to the left of, the atrioventricular septum for the pathway. This case, with the recent results of others, adds to the further understanding of accessory pathway location in W-P-W syndrome (type A).