Adults and 8-month-olds were presented with sequences in which every third complex tone was either longer or more intense. Segmentation was measured by comparing the detection of silent gaps inserted into three possible locations in each pattern: Silent gaps inserted at perceived segmentation boundaries are harder to detect than gaps within perceived phrases or groups. A go/no-go conditioned head-tum (hand-raising for adults) procedure was used. In Experiment 1, detection was worse for the gaps following the longer complex tones than for the gaps at the other locations, suggesting that the longer tones marked the ends of perceived groups for both infants and adults. Experiment 2 showed that an increase in intensity did not result in any systematic grouping at either age.The perception ofspeech and music involves analyzing the structure ofa pattern or stream of sounds that unfolds in time. One of the basic steps in this process is the segmentation of the stream into meaningful units or groups, such as notes, words, and phrases. For adults, it is likely that the segmentation of both music and language involves the use ofboth structural and acoustic cues. Structural cues in language include grammatical rules and phonotactic constraints. For example, some phonemes or groups of phonemes are more likely to begin words, whereas others are more likely to end words. Structural cues in music include the functional relations between the different notes of the scale. For example, musical compositions most often end on the tonic, or first note, of the scale. The acoustic cues to phrase endings in both