Brain-potential correlates of response conflict are well documented, but those of task conflict are not. Task-switching studies have suggested a plausible correlate of task conflict—a poststimulus posterior negativity—however, in such paradigms the negativity may also reflect poststimulus task-set reconfiguration postulated in some models. Here, participants alternated between single-task blocks of classifying letters and digits; hence, no within-block task-set reconfiguration was required. Presenting letters alongside digits slowed responses to the digits and elicited an ERP negativity from ∼350 ms, relative to task-neutral symbols presented alongside digits, consistent with task conflict. The negativity was also present for congruent digit-letter stimuli; this and the lack of behavioral response congruency effects indicate conflict at the level of task-set rather than response selection.