This paper reports on a longitudinal study of the acquisition of the voicing contrast in English word-initial stop consonants, as measured by voice onset time. Four monolingual children were recorded at two week intervals, beginning when the children were about 1;6. Data provided evidence for three general stages: (1) the child has no contrast; (2) the child has a contrast but one that falls within the adult perceptual boundaries of one (usually voiced) phoneme and thus is presumably not perceptible to adults; and (3) the child has a contrast that resembles the adult contrast. The rate and nature of the developmental process are discussed briefly in relation to two competing models for phonological acquisition and two hypotheses regarding the skills being learned. (Author)