The development of the use of the third-person singular -s in open syllable verbs in children with specific language impairment (SLI) and their typically developing peers was examined. Verbs that included overt productions of the third-person singular -s morpheme (e.g. Bobby plays ball everyday; Bear laughs when mommy buys popcorn) were contrasted with clearly bare stem contexts (e.g. Mommy, buy popcorn; I saw Bobby play ball) on both global and local measures of acoustic duration. A durational signature for verbs inflected with -s was identified separately from factors related to sentence length. These duration measures were also used to identify acoustic changes related to the omission of the -s morpheme. The omitted productions from the children with SLI were significantly longer than their correct third-person singular and bare stem productions. This result was unexpected given that the omitted productions have fewer phonemes than correctly inflected productions. Typically developing children did not show the same pattern, instead producing omitted productions that patterned most closely with bare stem forms. These results are discussed in relation to current theoretical approaches to SLI, with an emphasis on performance and speech-motor accounts.