Previous experiments (Rotberg & Woolman, 1963), in which similar and dissimilar stimulus groupings were compared, indicated the superiority of similar stimulus grouping. In those experiments, the similarity categories were clearly isolated during learning. In the present experiment, procedures were employed that provided a less marked separation of the similarity categories. Although the results ... [Show full abstract] confirmed the findings of the previous experiments in certain respects, similar stimulus grouping was not superior to dissimilar grouping. It is hypothesized that the superiority of similar stimulus grouping depends on the functional isolation of similarity categories.