Children aged 2;0 to 2;6 participated in a longitudinal study examining their acquisition of the English auxiliary system following a six-week period in which they were exposed to additional auxiliary input in varying sentence contexts. Groups of children received enrichment utterances with the auxiliary could either in first position in the sentence, in middle position, or in both positions. Children in the front position group were significantly advanced over the other experimental groups in acquiring modal auxiliaries but not non-modals. However, none of the experimental groups differed significantly from a baseline group which received no additional could input. The implications of these findings for understanding the mechanisms of auxiliary acquisition and the nature of children's grammatical categories are discussed.