Reaction times and event-related potentials in correct and incorrect trials were studied in a bimanual choice reaction task. In a focused attention (FA) condition, the stimulus modality was constant (visual or auditory); in a divided attention (DA) condition, the modality was varied at random from trial to trial. Stimulus- and response-triggered averages were computed from the midline EEG leads. In error trials, the ERP amplitude was reduced in the P300 range (300-500 msec) and enhanced in the slow wave range (500-700 msec) compared to correct reaction trials. Difference plots between the ERPs (incorrect minus correct reaction trials) revealed a large fronto-central negativity ("NE") and a parieto-occipital "slow wave." These components appeared larger in the response-triggered averages. We believe that they reflect two different stages of error processing. After auditory stimuli the NE peaked much later for DA than for FA, which supports the idea of an asymmetrical allocation of processing resources to the disadvantage of the auditory modality in our DA condition.