This study investigated whether a person with profound mental retardation could effectively learn specific requests using two different communication modes, both including the same set of graphic symbols. It also sought to compare whether the participant showed a preference for one modality over the other and to determine whether the participant could functionally use different communication modes depending on the characteristics of the environment. The young adult subject used manual signs as his primary means of communication. The two communication devices studied included a portable picture board and a programmable voice output communication aid (VOCA) with an overlay containing the same graphic symbols as the board. Results indicated that the subject met criteria for both communication devices at two settings during intervention and kept the criteria during preference assessment and maintenance phases. The subject showed stronger preference for one modality over the other. Generalization measures indicate that the subject generalized the use of his preferred communication device (VOCA) to different settings and different communication partners and successfully requested items of his choice at a fast food restaurant. (28 references) (JDD)