We report a patient who underwent radiofrequency catheter ablation of a left posteroseptal atrioventricular (AV) Mahaim fiber with a marked longitudinal dissociation. During atrial pacing, Wenckebach-type atrioventricular block over the accessory pathway was observed with progressive preexcitation and no change in polarity of the delta waves. The AV conduction curve was discontinuous, with a distinct "jump-up" in local AV conduction time of 84 ms. The earliest ventricular activation was recorded from the posteroseptal portion of the mitral annulus, and the unipolar electrogram from a distal electrode had a high, steep deflection with uniphasic QS-like activity with 62 ms of local AV conduction time.