Five experiments using the Headturn Preference Procedure examined 15- and 18-month-old children's sensitivity to morphosyntactic dependencies in English. In each experiment, the children were exposed to two types of passages. Passages in the experimental condition contained a well-formed English dependency between the auxiliary verb is and a main verb with the ending -ing. Passages in the control condition contained an ungrammatical combination of the modal auxiliary can and a main verb with the ending -ing. In the experiments, the distance between the dependent morphemes was systematically varied by inserting an adverbial of a specified length between the auxiliary and main verbs. The results indicated that 18-month-olds are sensitive to the basic relationship between is and -ing, but that 15-month-olds are not. The 18-month-olds, but not the 15-month-olds, listened significantly longer to the passages with the well-formed English dependency. In addition, the 18-month-olds showed this preference for the well-formed dependency only over a limited domain of 1-3 syllables. Over domains of 4-5 syllables, they showed no significant preference for the experimental over the control passages. These findings indicate that 18-month-olds can track relationships between functor morphemes. Additionally, these findings are consistent with the hypothesis that 18-month-olds are working with a limited processing window, and that they are only picking up relevant dependencies that fall within this window.