Subjects listened to sentences with early closure (e.g.,When Roger leaves the house is dark) or late closure syntax (e.g.,When Roger leaves the house it's dark) and one of three prosodies: cooperating (coinciding prosodic and syntactic boundary), baseline (phonetically neutralized prosodic boundary), and conflicting (prosodic boundary at a misleading syntactic location). Prosodic manipulations were verified by phonetic measurements and listener judgments. Four experiments demonstrated facilitation in speeded phonosyntactic grammaticality judgment, end-of-sentence comprehension, and cross-modal naming tasks: Sentences with cooperating prosody were processed more quickly than those with baseline prosody. Three experiments showed interference: Sentences with conflicting prosody were processed more slowly than those with baseline prosody. All experiments demonstrated a processing advantage for late closure structures in the conflicting and baseline conditions, but no differences between syntactic types in the cooperating condition. Cross-modal naming results showed early syntactic effects due to both high-level and intermediate-level prosodic boundaries. We argue that the initial syntactic structure assigned to an utterance can be determined by its prosodic phonological representation.