Neuroimaging and computational modeling studies have led to the suggestion that response conflict monitoring by the anterior cingulate cortex plays a key role in cognitive control. For example, response conflict is high when a response must be withheld (no-go) in contexts in which there is a prepotent tendency to make an overt (go) response. An event-related brain potential (ERP) component, the N2, is more pronounced on no-go than on go trials and was previously thought to reflect the need to inhibit the go response. However, the N2 may instead reflect the high degree of response conflict on no-go trials. If so, an N2 should also be apparent when subjects make a go response in conditions in which no-go events are more common. To test this hypothesis, we collected high-density ERP data from subjects performing a go/no-go task, in which the relative frequency of go versus no-go stimuli was varied. Consistent with our hypothesis, an N2 was apparent on both go and no-go trials and showed the properties expected of an ERP measure of conflict detection on correct trials: (1) It was enhanced for low-frequency stimuli, irrespective of whether these stimuli were associated with generating or suppressing a response, and (2) it was localized to the anterior cingulate cortex. This suggests that previous conceptions of the no-go N2 as indexing response inhibition may be in need of revision. Instead, the results are consistent with the view that the N2 in go/no-go tasks reflects conflict arising from competition between the execution and the inhibition of a single response.