The location of phonological phrase boundaries was shown to affect lexical access by English-learning infants of 10 and 13 months of age. Experiments 1 and 2 used the head-turn preference procedure: infants were familiarized with two bisyllabic words, then presented with sentences that either contained the familiarized words or contained both their syllables separated by a phonological phrase boundary. Ten-month-olds did not show any listening preference, whereas 13-month-olds listened significantly longer to sentences containing the familiarized words. Experiments 3 and 4 relied on a variant of the conditioned head-turning technique. In a first session, infants were trained to turn their heads for an isolated bisyllabic word. In the second session, they were exposed to the same sentences as above. Both 10- and 12.5-month-old infants turned significantly more often when the target word truly appeared in the sentence. These results suggest that phonological phrase boundaries constrain on-line lexical access in infants.