How are we to understand that Langevin, who has been a fellow traveller of the PCFParti communiste français (PCF) since its creation in 1920, who worked shoulder to shoulder with PCF members in several movements, and who frequented Communist circles, only joined the Party in his late career, when he was over 70? It was neither a lack of courage that prevented him from joining earlier, because he has taken many other risks. It is not a lack of contacts either, as there were already many communist activists among his children. Finally, it is not out of mistrust for the Soviet Union, since he has never formulated the slightest criticism of this regime. Against the testimony of close friends who claimed that Langevin was convinced long before his membership of the validity of the PCF political line, we argue that despite years of fighting side by side, Langevin kept a distance and did not always follow the line of the Party. We conclude that it is equally simplistic to label Langevin as a communist scientist as to argue that he has been instrumentalized by the PCF. His relations with the PCF were complex.