The purpose of this study was to examine the temporal pattern of pupils' approach-avoidance achievement goal adoption in physical education across Key Stage 3 of secondary school. Moreover, we determined the predictive utility of implicit theories of ability and perceived competence in explaining change in achievement goals, along with the moderating influence of pupils' year group. On four occasions, over a 9-month period, 511 pupils in Years 7, 8, and 9 completed measures of perceived competence, incremental and entity beliefs, and approach-avoidance goals. Mastery-approach, mastery-avoidance, and performance-avoidance goals exhibited a linear decline over time, whereas performance-approach goals showed no significant change. Theoretical propositions regarding the antecedents of approach-avoidance goal adoption were supported. Year group was found to moderate a number of these antecedent-goal relationships. Results suggest that Year 7 is a critical time for adolescents' motivation in school physical education. (Contains 4 tables.)