In the Fresnes jail, prisoners practice karate. If the benefits of sports in jail are well-established, what about the pedagogical effects of practicing a combat sport in prison? By comparing 188 observations carried out in an associative setting and 77 in detention, we analyze the aggressive motor conducts in karate in order to verify whether this practice brings a regulation of affects. These observations grade aggressiveness’ lawfulness on a scale ranging from minus two to plus two (on the basis of objective behavioral indicators of the degree of aggressiveness). Challenging conventional wisdom, the results show that members of karate associations are more aggressive than jailed karatekas. As far as the civility processes are concerned, they are expressed by a minus 2 average for the members and 0.81 for the inmates. Can karate have its place in prison as a sports activity, then? The answer seems to be yes. However, this activity cannot solve the rehabilitation problem on its own.