A 37-year-old man was referred to our institution for ablation of a cavotricuspid isthmus (CTI)-dependent atrial tachycardia (AT). The AT was terminated by linear ablation to the CTI. However, the same AT recurred, and a second procedure was performed. A surface electrocardiogram showed flutter waves with a negative deflection in the inferior leads (Fig. 1A). To assess the detailed activation sequence of the AT, two 20-pole deflectable electrode catheters were placed in a parallel position on the tricuspid annulus (TA) and right atrial lateral wall (LW) (Fig. 1B–D). During AT (cycle length: 280 ms), counterclockwise activation was observed in both the catheters, and a gap with conduction delay was observed on the anterior part of the CTI block line (Fig. 2A). During entrainment pacing from the lateral TA, the LW was captured one cycle later than the TA (Fig. 2B). In contrast, during entrainment pacing from the proximal coronary sinus (CS), the TA and LW were captured simultaneously (Fig. 2C). AT was terminated during radiofrequency application to the gap on the CTI block line. After the termination of AT, the completion of the CTI block line was confirmed. AT did not recur after the procedure. What is the mechanism of the delayed capture of the LW during entrainment pacing from the lateral TA?