This study examined the effects of increased autonomy on (a) self-determination and (b) physical activity levels. Seventh- and eighth-grade girls (N = 122) in four classes participated in two fitness units (one allowing choice of activities, the other no-choice). The order of the units was counterbalanced, so that two classes participated in the choice unit first, and the other two participated in the no-choice unit first. The abridged Situational Intrinsic Motivation Scale (SIMS) was administered after each unit. Pedometers were used to measure step counts during both units. Overall, self-determination was higher in the choice unit. The repeated measures analysis also indicated that girls who experienced the choice unit first, and then were denied the opportunity to make choices had the lowest levels of self-determination. The results provide empirical support for the theoretical prediction that increased autonomy yields higher levels of self-determination. (Contains 1 figure and 3 tables.)