Infants of 18 and 24 months acquiring English were tested in a preferential looking task on their ability to detect ungrammaticalities caused by manipulating a single function word in sentences. Infants heard grammatical sentences in which the determiner the preceded a target noun, as well as three ungrammatical conditions in which the was either dropped, replaced by a nonsense function word (el), or replaced by an alternate English function word (and). Both the 18- and 24-month-old infants oriented faster and more accurately to a visual target following grammatical sentences. The results suggest that by 18 months of age, infants use their knowledge of determiners in sentence computation and in establishing reference.