Trainer behavior has been neglected as an area of applied research. Feedback has been demonstrated as potentially effective to increase the accuracy of behavior of teachers, parents, ward-staff members, and other service providers in the area of mental retardation. In this study, we assessed the effect of delayed feedback on the accuracy of training given by four trainers during one-to-one sessions. Trainees were four individuals with severe mental retardation, who were taught communicative gestures. The results, gathered within a multiple baseline design across dyads of trainers and trainees, showed that delayed feedback resulted in an increased accuracy of training.