To cope with the new challenges inherent to their political role, Civil Society Organizations (CSOs) must convince their stakeholders about their legitimacy, and meta-organizations (MOs) appear to play a central role in such a context (Arhne & Brunsson, 2005; Bonfils, 2011). In this paper, we aim to better understand the legitimating processes of a specific kind of MOs-namely Civil Society M Os (CSMOs)-, considering that CSMOs feature some characteristics that reinforce both internal and external legitimacy issues. Our research is based on an in-depth case study of a French national federation (Fédération Addiction) formed by the merger of two former federations originating in different fields, alcoholism treatment and drug addiction professionals. We confirm the importance of stakeholders' representativeness in the governance of MOs and especially in multi-stakeholders CSMOs, and we corroborate the assertion that MOs closely relate to categorization-related issues and the categorization process itself in many ways: the legitimacy and the potential f or action of M Os depend on the socially perceived appropriateness of the delimitation of the f ield that they claim to represent, and at the same time categorization is reinforced by the creation of MOs. We contribute to the current literature on MOs in two main ways. First, we show how a change in the relevant categorization may result from the dual and interacting actions of the M Os themselves and public authorities. Second, our case study illustrates how a restructuring of the MOs landscape may strengthen the salience of internal legitimacy issues federative actors are confronted with in order to maintain their representativeness and position in the expanded organizational field. In this dynamic context, external and internal legitimating processes appear closely intricate, and categorization and governance issues appear strongly interrelated.