It has been demonstrated that the Biosense Carto system can improve the success rate of ablation in case of an intra-atrial reentry tachycardia in patients submitted to the Mustard repair. This system was used to map an intra-atrial reentry tachycardia in a young patient who had been submitted to the Mustard procedure for atrial situs inversus. A line of block was created connecting the right sided tricuspid valve to the left sided inferior vena cava. This terminated the arrhythmia and prevented its re-initiation. This case confirmed the notion that the cavo-tricuspid isthmus is often critical to the maintenance of an intra-atrial reentry tachycardia after the Mustard procedure even if its location is in the inversus side.