The role of mathematical interest, identity, utility, self-efficacy, and effort was examined as a set of interdependent factors leading to students' mathematics achievement. A structural equations model, testing a hypothesized structure of motivation variables and their impact on middle school mathematics achievement was developed utilizing the High School Longitudinal Study of 2009 (National Center for Education Statistics, 2011). Results support the theory developed by Middleton Leavy, and Leader (in press), emphasizing the central role of interests in determining effort, utility and self-efficacy. However, the relatively small amount of variance accounted for by the model in predicting mathematics achievement indicates that other, unresearched intervening variables must be acting on the system. Implications for research on motivation and design of interventions are discussed.