Language has a rhythmic structure, but little is known about the mechanisms that underlie how it is planned. Traditional models of language production assume that metrical and segmental planning occur independently and in parallel (Roelofs & Meyer Learning Memory and Cognition, 24(4), 922-939, 1998). We test this claim in two experiments. In Experiment 1, participants completed an event-description task in which a disyllabic target word shared segmental overlap with a prime that either had matching or nonmatching lexical stress. Participants lengthened words in trials with both segmental and metrical overlap, which could either be the result of metrical interference or having uttered a prime with similar segmental realizations. To adjudicate between these possibilities, Experiment 2 included segmentally distinct word pairs with either matching or nonmatching stress. Participants again showed lengthening in trials with both segmental and metrical overlap, but no lengthening from metrical overlap alone. These data suggest that the acoustic-phonetic similarity of the initial syllables of the prime and target creates competition that leads to word lengthening. These are consistent with production models in which segmental and metrical structures are tightly bound at the point of phonological encoding.