A series of four experiments examined infants' capacities to detect repeated words in fluent speech. In Experiment 1, 7 1/2-month old American infants were familiarized with two different monosyllabic words and subsequently were presented with passages which either included or did not include the familiar target words embedded in sentences. The infants listened significantly longer to the passages containing the familiar target words than to passages containing unfamiliar words. A comparable experiment with 6-month-olds provided no indication that infants at this age detected the target words in the passages. In Experiment 3, a group of 7 1/2-month-olds was familiarized with two different non-word targets which differed in their initial phonetic segment by only one or two phonetic features from words presented in two of the passages. These infants showed no tendency to listen significantly longer to the passages with the similar sounding words, suggesting that the infants may be matching rather detailed information about the items in the familiarization period to words in the test passages. Finally, Experiment 4 demonstrated that even when the 7 1/2-month-olds were initially familiarized with target words in sentential contexts rather than in isolation, they still showed reliable evidence of recognizing these words during the test phase. Taken together, the results of these studies suggest that some ability to detect words in fluent speech contexts is present by 7 1/2 months of age.