Previous studies have shown that infant-directed speech (IDS) differs from adult-directed speech (ADS) on a variety of dimensions. The aim of the current study was to investigate whether acoustic differences between IDS and ADS in English are modulated by prosodic structure. We compared vowels across the two registers (IDS, ADS) in both stressed and unstressed syllables, and in both utterance-medial and -final positions. Vowels in target bisyllabic trochees in the speech of twenty mothers of 4- and 11-month-olds were analyzed. While stressed and unstressed vowels differed between IDS and ADS for a measure of F0, and trended in similar directions for vowel peripherality, neither set differed in duration. These profiles held for both utterance-medial and -final words.