Thumbsucking of a girl aged 2 years and 9 months was recorded by the parents during three 10-minute reading baselines. In Study 1, thumbsucking was punished by contingent termination and resumption of reading. In Study ii, positive reinforcement was provided for other (non-thumbsucking) behavior. The results imply that parents can effectively control the thumbsucking of a very young child in the home by differential reinforcement of non-thumbsucking behavior. (CS)