The use of finiteness markers copula BE, auxiliary BE, and auxiliary DO were examined in the spontaneous speech of children with specific language impairment (SLI). Of particular interest to this study was whether the categorical distinctions between main verbs and auxiliaries and/or between the auxiliary types influenced the relative order of emergence among these forms. In addition, error analyses were used to reveal the extent of the children's grammatical knowledge with regard to the use of these forms. We argue that despite the late emergence of finiteness markers, children with SLI demonstrate knowledge of how finiteness interacts with verb movement and accurate agreement marking from the earliest appearance of these forms. These findings provide further support for descriptions of SLI as a condition characterized by selective deficits within a basically intact grammatical system.