Whenever children hear a novel word, the context supplies information about its meaning. One way children may cope with so much information is to use whatever seems to make sense, given their prior knowledge and beliefs, while ignoring or quickly forgetting the rest. This work examined if and how children's beliefs about word meanings may affect their use of contrastive linguistic information in the input in word learning. In Study 1, some 3- and 4-year-olds were introduced to a novel material or shape name and heard it contrasted with familiar words. Others merely heard the novel word used for referring to an object. These children were then tested to determine what they had learned about their new word meaning. In Study 2, another group of 3-and 4-year-olds were asked to name the materials and shapes used for introducing these novel terms. Children made use of linguistic contrast only in some situations. They benefited more when the novel term did not overlap much in denotation with any terms commonly known by 3-and 4-year-olds. These results suggest that children can use information in the input very efficiently in learning a term for an as-yet-unnamed category, but not in learning a term similar in denotation to a word they already know. Thus, the results are consistent with the claim that children believe every word has a unique denotation.