This study employed a prospective questionnaire design to assess the predictive validity of prototypes and descriptive norms in relation to young people's exercise behaviour after variables from the theory of planned behaviour (TPB) and past behaviour had been controlled. Findings supported the utility of the TPB, descriptive norms, prototype similarity, and past behaviour in predicting intentions and behaviour. Importantly, prototype similarity was directly associated with behaviour, both on its own and through its relationship with descriptive norms, even after controlling for the TPB and past behaviour. Findings are discussed in relation to; (1) the role of social identification in motivating health-related decisions and behaviour, (2) the current conceptualisation of social influences in the TPB, and (3) recent research on the "perception-behaviour expressway".