Previous research on acoustic cues responsible for the voiced-voiceless distinction in prestressed English plosives has emphasized the importance of voicing onset time with respect to plosive release (VOT). Voiced plosives in English normally have a short VOT (less than 20–30 msec) and a significant formant transition is present following voice onset. Voiceless plosives in prestressed position, on the other hand, have relatively long VOT's (greater than about 50 msec) and the formant transitions are essentially completed prior to voice onset. Our experiments with synthetic speech compare the role of VOT and the presence or absence of a significant formant transition following voicing onset as cues for the voiced-voiceless distinction. The data indicate that there is a significant trading relationship between these two cues. The presence or absence of a rapid spectral change following voice onset produces up to 15-msec change in the location of the perceived phoneme boundary as measured in terms of absolute VOT. One can speculate that the auditory system may be predisposed to detect the presence or absence of a rapid spectrum change as a general property of acoustic inputs. If this is the case, then the acquisition of the voiced-voiceless distinction in infants may be conditioned initially by the presence or absence of this property at the onset of voicing rather than by absolute VOT.