Background: Atrial tachycardia (AT) occurring during atrial fibrillation (AF) ablation is sometimes difficult to identify and eliminate. EnSite Array (EA) visualizes beat-to-beat virtual activation of AT. The aim of the present study was to characterize AT occurring during AF ablation during ongoing AF, using EA. Methods and Results: Among 90 patients with AF (paroxysmal, n=67; persistent, n=23) who underwent radiofrequency catheter ablation during ongoing AF, 33 (37%) had 46 ATs that developed during ablation, and 9 (10%) of these patients had 9 ATs that developed before ablation. AT was sustained in 39 and non-sustained in 7. Nineteen ATs resulted from a focal mechanism and 27 from macroreentry. The major AT foci were distributed in the pulmonary vein (n=8) and left atrial roof (n=3), and macroreentrant ATs mainly consisted of peri-mitral AT (n=10), common atrial flutter (n=10), and roof reentrant AT (n=3). After EA-guided ablation of AT, 41 ATs in 28 patients (85%) were eventually rendered non-inducible. During 21±8 months of follow-up, 30 of the 33 patients (91%) were free from AF/AT recurrence. Conclusions: AT occurred in 37% of the patients during ongoing AF ablation, resulting from a focal or reentrant mechanism in diverse locations. Peri-mitral AT, common atrial flutter, and AT from the pulmonary vein were frequently observed. These ATs were eliminated by EA-guided radiofrequency ablation in most cases. (Circ J 2011; 75: 1080-1089)