This study was designed to characterize the brain system that monitors speech in people who stutter and matched controls. We measured two electrophysiological peaks associated with action-monitoring: the error-related negativity (ERN) and the error positivity (Pe). Both the ERN and Pe were reliably observed after errors in a rhyming task and a nonverbal flanker task, replicating previous reports of a language-monitoring ERN and demonstrating that the Pe can also be elicited by phonological errors. In the rhyming task, stutterers showed a heightened ERN peak regardless of whether they actually committed an error. Similar results, though only marginally significant, were obtained from the flanker task. These results support the vicious cycle hypothesis, which posits that stuttering results from over-monitoring the speech plan. The elevation of the ERN in stutterers and the similarity of the results between the flanker and rhyming tasks implies that speech-monitoring may rely on the same neural substrate as action-monitoring.