Thirty-one children with specific language impairment participated in 48 intervention sessions designed to assist them in the use of 3rd-person singular -s or auxiliary is/are/was. Gains in the use of these target forms were significantly greater than gains on developmentally comparable morphemes serving as control forms. Untreated verb forms that mark both tense and agreement showed greater change during the intervention period than did past -ed. The findings suggest that by gaining skill in the use of morphemes that mark both tense and agreement, the children were able to identify and acquire other morphemes in the language that mark both of these features. This increase in sensitivity did not appear to apply to forms in the language that express tense only.