The goal of this paper is to introduce a new measure of basic-level performance that we will call the "category attentional slip." The idea behind it is very simple: The attentional mechanisms of an ideally rational categorizer are made to "slip" once in a while. We provide a formalization of attentional slip that specifies what an "ideally rational categorizer" is and how its attention "slips." We then compare its predictive capabilities with those of two established basic-level measures: category feature-possession (Jones, 1983) and category utility (Corter & Gluck, 1992). The empirical data used for the comparisons are drawn from eight classical experiments from Murphy and Smith (1982), Murphy (1991), and Tanaka and Taylor (1991).