Fluent speech does not contain obvious breaks to word boundaries, yet there are a number of cues that listeners can use to help them segment the speech stream. Most of these cues have been investigated in isolation from one another. In previous work, Norris, McQueen, Cutler, and Butterfield (1997) suggested that listeners use a Possible Word Constraint when segmenting fluent speech into individual words. This constraint limits the word recognition system to consider only those parsings that could conceivably be words in the language (that is, those that do not strand illegal sequences). The present paper examines how this constraint interacts with other cues to segmentation, such as junctural and allophonic cues and neighborhood probabilities. Segmentation was influenced both by the PWC and by the presence of acoustic cues to juncture, such as the acoustic results of a speaker's intention to produce a particular phoneme as the end of one syllable vs. as the start of another (vuff-apple vs. vuh-fapple). In contrast, segmentation was not affected by the legality of a syllable-final vowel (tense vs. lax), or by the similarity of a sequence to words. This suggests that acoustic cues in the signal play a far larger role in segmentation than do sources of bias from the lexicon, and that probabilistic lexical information from the lexicon (such as neighborhood information) is unlikely to be used in the process of word segmentation.