This study used elicited production methodology to investigate the negative sentences that are produced by English-speaking children with specific language impairment (SLI). Negative sentences were elicited in contexts in which adults use the negative auxiliary verb doesn't (e.g., "It doesn't fit"). This form was targeted to see how negative markers ("not", "n't") interact with the 3rd person singular -s in the grammars of children with SLI. A robust sample of negative sentences was collected from 63 children: 21 5-year-old children with SLI, 21 language-matched typically-developing children and 21 age-matched typically-developing children. The negative sentences produced by children in the two control groups were consistently adult-like. In contrast, only 5 of the 21 children in the SLI group produced adult-like negative sentences with "doesn't". The other 16 children with SLI produced a variety of non-adult negative sentences (e.g., "It not fit", "It's not fit", "It not fits", "It's not fits"). The study concludes that children with SLI undergo an extended period in the development of sentential negation during which they produce a range of non-adult sentences that are similar to those previously reported in younger 2- to 3-year-old typically-developing children.