We report radiofrequency ablation of a Mahaim fiber in a patient with wide complex supraventricular tachycardia. Pathway potentials from the lateral aspect of the right AV groove were recorded, which were distinct from the His potential. During atrial pacing, decremental properties of the fiber were demonstrated, which resulted in prolongation of the interval between the atrial electrogram and the Mahaim pathway potential. The pathway potentials, preexcitation, and tachycardia disappeared after a single application of radiofrequency energy, after which the patient has remained free of palpitations. Mapping of a Mahaim fiber by identifying pathway potentials thus allowed accurate localization and successful ablation with minimal energy. We therefore suggest that, where possible, recording of such Mahaim potentials may be the optimal technique for Mahaim fiber localization.