One common outcome of using simultaneous communication to train the expressive use of manual signs is overselective attention to the visual component of discriminative stimuli for manual signing. Consequently, mentally retarded children may not acquire speech comprehension as a result of such training, despite the many occasions on which they experience the co-occurrence words and their referents. This study used an alternating treatment comparison to assess the relative efficacy of two methods of overcoming selective attention during simultaneous communication training. The first method, Differential Sign Training, involved mixing simultaneous communication trials with an equal number of trials in which the cue for manual signing was the referent word alone. The second method, Extensive Sign Training, involved overtraining expressive manual signing in simultaneous communication, but with a reduced reinforcement schedule. Although the Extensive condition produced the faster acquisition of expressive manual signing, only the Differential condition was successful in overcoming overselective attention, and thus facilitating speech comprehension.