Purpose: Our purpose was to test the competing sources of input (CSI) hypothesis by evaluating an intervention based on its principles. This hypothesis proposes that children's use of main verbs without tense is the result of their treating certain sentence types in the input (e.g., "Was 'she laughing'?") as models for declaratives (e.g., "She laughing"). Method: Twenty preschoolers with specific language impairment were randomly assigned to receive either a CSI-based intervention or a more traditional intervention that lacked the novel CSI features. The auxiliary "is" and the third-person singular suffix -"s" were directly treated over a 16-week period. Past tense -"ed" was monitored as a control. Results: The CSI-based group exhibited greater improvements in use of "is" than did the traditional group (d = 1.31), providing strong support for the CSI hypothesis. There were no significant between-groups differences in the production of the third-person singular suffix -"s" or the control (-"ed"), however. Conclusions: The group differences in the effects on the 2 treated morphemes may be due to differences in their distribution in interrogatives and declaratives (e.g., "'Is' he hiding/He 'is' hiding" vs. "'Does' he hide/He hide's'"). Refinements in the intervention could address this issue and lead to more general effects across morphemes.