This research aims at understanding the process of internal adaptation of an organization. This process, based on the implementation of a written regulation that is modified by the actors, leads to mobilize the theory of the social regulation (Reynaud). The internal adaptation is analyzed as a process of complex regulation. Qualitative methodology is based on a single case study analyzing the evolution of an association of the social sector in France.The main teachings concern the long-term adaptation of the organization. The creation process of a rule within the framework of the adaptation and the place of the various regulations in this dynamics, are brought to light.