The major aim of this article is to examine how migrations affect private governance, taking as a case study the Prud'homie de pêche, a private order that has governed the fishery of Marseille for the past six centuries. Scholarship generally argues that social norms guarantee the efficiency of private orders and their ability to resist the arrival of newcomers. My data suggest that the Prud'homie has failed to accommodate social changes prompted by migratory flows, not despite but because of its social norms. This paper suggests that social norms are not only powerful tools of governance for private orders, but also forces of inertia that can prevent these orders from accommodating social changes. In the past few decades, an influential stream of scholarship has argued that "private orders" develop self-governance mechanisms in order to solve collective action problems and that human societies can benefit greatly from these mechanisms (Bernstein 1992; Ellickson 1991; 2007; Richman 2017; Stringham 2015). Although most of this scholarship has been undertaken by law and economics scholars, its deeper roots can be found in sociolegal scholarship that has examined the importance of social norms for the governance of human societies (