A typical mathematics assignment consists primarily of practice problems requiring the strategy introduced in the immediately preceding lesson (e.g., a dozen problems that are solved by using the Pythagorean Theorem). This means that students know which strategy is needed to solve each problem before they read the problem. In an alternative approach known as "interleaved practice," problems from the course are rearranged so that a portion of each assignment includes different kinds of problems in an interleaved order. Interleaved practice requires students to choose a strategy on the basis of the problem itself, as they must do when they encounter a problem during a comprehensive examination or subsequent course. In the experiment reported here, 126 seventh-grade students received the same practice problems over a three-month period, but the problems were arranged so that skills were learned by interleaved practice or by the usual blocked approach. The practice phase concluded with a review session, followed 1 or 30 days later by an unannounced test. Compared to blocked practice, interleaved practice produced higher scores on both the immediate and delayed tests (Cohen's d = 0.42 and 0.79, respectively). Two appendices include: (1) Serial Position of Each Graph and Slope Problem in the Assignments (table); and (2) Frequency of Responses of Three Teachers to Statements About Interleaved Practice (table). [Note: This article was "in press" at the time of submission. No citation information is available at this time.]