The roles of N100 and the early and late negative difference (Nd) waveforms in selective attention were investigated with primary simulated flying and secondary dichotic listening tasks. Twenty highly trained participants performed the two tasks together, either while detecting secondary task deviant tones in one ear and ignoring tones in the other ear (dual condition) or while ignoring the tones in both ears (ignore condition). The amplitude of the N100-P200 complex in both the dual-task and ignore conditions was reduced equally in the attended and unattended ear by the introduction of the primary task. In the dual-task condition, the amplitude of the late Nd but not the early Nd was reduced by a combination of the introduction of the primary task and an increase in its difficulty. P300 showed the same amplitude pattern as the late Nd. We propose that the reduction in N100-P200 amplitude reflects an automatic gating mechanism. The dissociation of the early and late Nd suggests that the latter is not modality specific. The question is raised whether the late Nd is more closely associated with P300 than with the early Nd.