The main purpose of this pilot study was to examine the relations between estimated biological maturity status, body size, and leisure-time exercise behaviour in British Year 9 students. Estimated biological maturity status and body size were assessed in 103 male and 82 female students aged 13-15 years (mean 14.04 years, s = 0.31) in physical education classes. To assess exercise behaviour, participants completed the Leisure-Time Exercise Questionnaire (Godin & Shephard, 1985). One year later, 127 of the participants were reassessed for maturity status and body size. Maturity status was positively correlated with involvement in mild forms of reported exercise behaviour and total reported exercise activity in males, and negatively associated with involvement in strenuous forms of reported exercise in females. Hierarchical regression models, with main and interactive effects of body size and maturity status as predictor variables, failed to predict a significant proportion of variance in total reported exercise activity in both males and females.