The role of negative instances in the acquisition of the mathematical concepts of distributivity and homomorphism was examined. Two treatment levels for distributivity (positive instances and positive and negative instances) and the same treatment levels for homomorphism were crossed to form a 2 x 2 factorial design with 23 subjects per cell. Criterion variables were number of correct classifications, stimulus intervals, and postfeedback intervals. All pretests, threatments, and posttests were administered using computer terminals. There was a significant effect for treatment levels of distributivity on the correct classifications of new instances of distributivity favoring the treatments containing negative instances (p .05). There was a significant interaction between treatment levels of distributivity and treatment levels of homomorphism on correct classification of new instances of homomorphism (p .05). The results supported the hypotheses that negative instances enhance mathematical concept acquisition and that effects of negative instances for one concept transfer to another concept. (Author/DT)