The present study examined how beliefs about intelligence, as mediated by ability-validation goals, predicted whether students lost or maintained levels of intrinsic motivation over the course of a single academic year. 978 third- through eighth-grade students were surveyed in the fall about their theories concerning the malleability of intelligence, need to validate their academic ability through schoolwork, and intrinsic motivation. At the end of the school year, they were surveyed again about their intrinsic motivation and subsequently characterized as either decliners (those who lost intrinsic motivation over the year) or maintainers (those who maintained or gained intrinsic motivation over the year). As predicted, decliners were more likely to endorse an entity theory of intelligence than maintainers and this relationship was fully mediated by the adoption of ability-validation goals. Implications for intervention efforts and future research are discussed. (Contains 2 figures and 2 tables.)