The aims of this study were to examine the genetic contribution on physical activity (PA) within a nationwide population of young adult male twin pairs from Sweden and to investigate the equal environment assumption (EEA) in relation to PA. Information on PA was collected by questionnaires in 1998 and 2002 and the impact of genetic factors was estimated by structural equation modeling (SEM). The study included 1,022 pairs of twins and the best fitting SEM-model gave a heritability of 49% (95% CI, 40-56%) for total PA and all PA dimensions showed genetic contributions between 40% and 65%. Non-shared environmental factors were also important, whereas shared environmental factors did not contribute to PA behaviors. The EEA was investigated with a linear regression model, examining if the twins contact frequency predicted within-pair differences in PA, and further by a simulation study. We found no support for violation of the EEA.