Linguistic and perceptual arguments suggest that, in speech recognition systems, syntactic hypotheses should be formed before phonemic segments are identified. Prosodic features can provide some cues to constituent structure. In a variety of texts and excerpts from conversations, spoken by several talkers, a decrease in voice fundamental frequency (F0) usually occurred at the end of each major syntactic constituent, and an increase in F0occurred near the beginning of the following constituent. A computer program based on this regularity correctly detected over 80 percent of all syntactically predicted boundaries. Some boundaries between minor constituents were also detected by the fall-rise patterns in F0. False boundary detections resulted from F0variations at boundaries between vowels and consonants, but most such false alarms could be eliminated by setting a minimum percent variation in F0for a boundary detection. Sentence boundaries were accompanied by large F0increases and substantial pauses. The categories of constituents affect boundary detection results, with noun phrase-verbal sequences showing particularly infrequent detection. Prosodic cues to stress patterns and stress-to-syntax rules may be used to detect other aspects of syntactic structure. Syntactic structure hypotheses might then be used to guide phonetic recognition procedures within constituents.