This article addresses native monolingual American and Mandarin Chinese incipiently bilingual children's ability to detect and identify inter- and intraword boundaries. Two hypotheses are reported: first, that young children will demonstrate similar patterns in their segmentation behavior, and that there will be a developmental progression in this behavior; and second, that Mandarin Chinese subjects learning to read Chinese and English simultaneously will segment English words more readily into syllables than American subjects, treating them as Chinese monosyllables (C)V(C). Although results partially support the first hypothesis, no group main effect was found. However, when the same subjects' intraword segmentation patterns were further analyzed to probe their awareness of adult phonological constraints while spontaneously segmenting words into (C)VC or CV(C) units, there was evidence of difference in group segmentation patterns. With time, though, the bilingual subjects appear to develop nativeike phonological constraints.