We report a patient in whom mapping of the right atrium with multipolar catheters and electroanatomic mapping revealed the presence of three dissimilar rhythms: a reentrant atrial tachycardia in the antero-lateral wall of the right atrium and an atrioventricular nodal reentrant tachycardia (AVNRT) isolated from each other and a conduction disturbance at the interatrial septum resulting in a rate-related interatrial block and a slow left atrial rhythm. The AVNRT was stopped with intravenous adenosine (6 mg) and induced repeatedly by atrial extrastimuli associated with a critical atrioventricular delay and dual atrioventricular nodal pathways. Electroanatomic mapping disclosed extensive fibrosis isolating viable myocardium of the antero-lateral wall from the rest of the right atrium. The viable myocardium in the antero-lateral wall was activated by a reentrant rhythm circulating around an islet of fibrosis located in the middle of the viable tissue. The AVNRT was ablated by a standard approach and the reentrant atrial tachycardia by producing a linear lesion bridging the central islet of fibrosis with the anterior tricuspid annulus. This case highlights the complicated nature of some dissimilar atrial rhythms and the power of electroanatomic mapping tools to reveal the exact mechanism and guide radiofrequency ablation.