We present a probabilistic model that uses both prosodic and lexical cues for the automatic segmentation of speech into topic units. The approach combines hidden Markov models, statistical language models, and prosody-based decision trees. Lexical information is obtained from a speech recognizer, and prosodic features are extracted automatically from speech waveforms. We evaluate our approach on the Broadcast News corpus, using standard evaluation metrics. Results show that the prosodic model alone outperforms the word-based segmentation method. Furthermore, we achieve an additional reduction in error by combining the prosodic and word-based knowledge sources.