Three studies investigated the perception of rhythmic units in speech to explore the potential role of rhythm in word-level segmentation. Experiment 1 investigated whether adults expect trochaic (strong-weak) units to cohere. The prediction that poststress pauses, which violated the coherence of the trochee, would be unexpected and therefore more noticeable was supported: Subjects were consistently more likely to identify post-than prestress pauses in trisyllabic speech stimuli. In Experiment 2, a variant of a head-turn preference procedure was used to test a similar question with 7-and 9-month-old infants. Nine-month olds showed a significant preference for stimuli in which the coherence of the trochee was preserved, suggesting that they perceived those stimuli as more natural than stimuli in which trochaic sequences were disrupted by pauses. Among 7-month olds, no preferences were observed. A third experiment tested the possibility that 9-month olds use the trochaic stress pattern in word-level segmentation. Subjects distinguished a trochaic sequence that had previously been embedded in a four-syllable string from a novel trochaic sequence, suggesting that they recognized the previously heard trochaic unit as familiar. No such discrimination was present for iambic targets and distracters, suggesting that infants failed to recognize the previously heard iambic sequence.