People alter their task performance on a trial-to-trial basis, for example after an incongruent trial on tasks involving response conflict. Previous research has found that these adjustments are most robust in the Simon task. One explanation for behavioral adjustments is the conflict-monitoring hypothesis, which posits that the dorsal anterior cingulate cortex (ACC) responds to conflict and that this serves as a signal to recruit other brain regions such as the prefrontal cortex (PFC) to minimize conflict and improve performance. However, another independently supported explanation for behavioral adjustments on the Simon task is the feature integration view, which can account for behavioral adjustments as the result of stimulus repetitions and alternations. Hence, by itself, evidence for behavioral adjustments on the Simon task does not clearly provide evidence for the conflict-monitoring hypothesis. However, the conflict-monitoring hypothesis does predict that behavioral adjustments on the Simon task should involve ACC conflict activity and PFC post-conflict activity. In the current study, consistent with the conflict-monitoring hypothesis, behavioral adjustments in performance on the Simon task were predicted by ACC conflict-related activity. In addition, subsequent behavioral adjustments were associated with PFC activity, with previous trial ACC conflict-related activity predicting greater PFC activity on subsequent trials. These results provide additional evidence that behavioral adjustments on the Simon task are due in part to ACC conflict monitoring and the subsequent recruitment of PFC to minimize conflict.