The discrimination-reversal skills of groups of Cercopithecus and Lemur were studied to determine whether this learning-set relevant skill might be validly assessed with a totally automated system and data analysis method designed to ensure equity for these diverse primate forms. The relative + and — cue values of a single pair of projected stimuli were reversed on 200 successive 51-trial units. The groups differed significantly in reversal performances, even when equated for acquisition proficiency, in accord with predictions based on the groups’ relative phyletic levels.