The current study addresses an aspect of second language (L2) phonological acquisition that has received little attention to date--namely, the acquisition of allophonic variation as a word boundary cue. The role of subphonemic variation in the segmentation of speech by native speakers has been indisputably demonstrated; however, the acquisition of allophonic cues in L2 phonology remains underexplored. We examine here L2 learners' acquisition and perception of noncontrastive acoustic differentiation at word boundaries in English. Fifty French-speaking students of English were tested on their ability to differentiate potentially ambiguous phrases in which word boundaries are marked by the word-initial aspiration of plosives (e.g., "Lou stops" vs. "loose tops") or prevocalic glottal stops (e.g., "tea mat" vs. "team at"). Participants showed greater sensitivity to the presence of glottal stops than aspiration, suggesting that glottal stops may represent a more perceptually salient segmentation cue for learners than aspiration. We discuss the implications of these results regarding the role of first language transfer versus the universality of some segmentation cues.