This study estimated the effect of extraversion, conscientiousness and variables from a modified two-component Theory of Planned Behaviour (TPB) on prospective self-reported physical activity (N = 397). Mediation analysis using structural equation modelling with bootstrapping suggested that only conscientiousness had significant effects on intention and behaviour. Affective attitude and perceived behavioural control partially mediated the effect on intention. The effect on behaviour was significantly mediated by TPB variables consistent with partial or total mediation. The modified TPB mediates the effects of conscientiousness on intention and behaviour; it is unclear whether it is sufficient to mediate all of the effect on behaviour.