Across a variety of tasks, adults respond differently to syllables with multiple stress cues than to syllables with only one cue to stress. This series of experiments was designed to explore how infants and adults use partial stress as a cue to word boundaries. In the first experiment, 9-month-old infants treated syllables with only one cue to stress (spectral tilt) as a strong cue to word boundaries. The second experiment shows that whereas adults treat syllables with multiple cues to stress as word onsets, they do not consider syllables marked only by spectral tilt to be strong indicators of word boundaries. Thethird experiment shows that 1-year-old infants are more adultlike than 9-month-olds in their use of stress cues. Taken together, these results suggest a rapid development of stress cue knowledge in infancy, perhaps due to infants' experience with word segmentation.