This study provides a systematic demonstration of generalization failures in which irrelevant stimulus control of competing responses interferes with, or overrides, the stimulus control of adaptive behavior developed during training. Appropriate and inappropriate verbal responses by a 26-year-old woman with severe mental retardation served as the dependent variables. A reversal design across trained and nontrained settings indicates that presenting a stimulus with a prior history of control over "inappropriate" verbalizations concurrently with a stimulus controlling newly acquired "appropriate" verbalizations significantly reduces the strength of the appropriate stimulus-control relationship. The implications for training generalized skills are discussed.