A dissociation between categorization and similarity was found by Rips (1989). In one experiment. Rips found that a stimulus half-way between a pizza and a quarter was categorized as a pizza but was rated as more similar to a quarter. Smith & Sloman (1994) discuss these results in terms of the role of necessary and characteristic features. In one experiment, participants had to learn to categorize new stimuli (unknown shapes) built with necessary and characteristic features. We compared two experimental conditions in which we manipulated the association between the characteristic features and the two categories. Contrary to the suggestion made by Smith and Sloman, subjects categorized the stimuli on the basis of a necessary feature. However, their similarity judgments relied on the characteristic features. This resulted, for one of the two experimental conditions, in a perfect dissociation betweensimilarity and categorization. According to Rips, the dissociation indicates that categorization and similarity rating are different processes. On the contrary, we suggest that categorization and similarity are the same processes, but that they sometimes operate on different subsets of features.