Infants become selectively sensitive to phonological distinctions relevant to their native language at an early age. One might expect that infants bring some of this phonological knowledge to bear in encoding the words they subsequently acquire. In line with this expectation, studies have found that 14-month-olds are sensitive to mispronunciations of initial consonants of familiar words when asked to identify a referent. However, there is very little research investigating infants' sensitivity to vowels in lexical representations. Experiment 1 examines whether infants at 15, 18 and 24 months are sensitive to mispronunciations of vowels in familiar words. The results provide evidence for vowels constraining lexical recognition of familiar words. Experiment 2 compares 15, 18 and 24-month-olds' sensitivity to consonant and vowel mispronunciations of familiar words in order to assess the relative contribution of vowels and consonants in constraining lexical recognition. Our results suggest a symmetry in infants' sensitivity to vowel and consonant mispronunciations early in the second year of life.