The present study assessed the underlying mechanism of execution-related dual-task interference in the psychological refractory period (PRP) paradigm. The motor bottleneck hypothesis attributes this interference to a processing limitation at the motor level. By contrast, the response monitoring hypothesis attributes it to a bottleneck process that not only selects the appropriate response but also monitors its execution. In two experiments, participants performed ballistic movements of different distances in Task 1 and a choice reaction time task in Task 2. In each experiment, a propagation effect of movement distance on reaction time in Task 2 indicated substantial execution-related interference. To determine the locus of this effect, we manipulated stimulus-response compatibility in Task 2. In line with the motor bottleneck hypothesis, the compatibility effect was partially absorbed during movement execution of Task 1. The results support a motor bottleneck mechanism rather than response monitoring as the source of execution-related dual-task interference. (Contains 2 footnotes, 3 tables, and 5 figures.)