This paper deals with the case of the Mexican PRD (Partido de la Revolucion Democratica). It studies the organizational evolution of PRD using a model of institutionalizaton which diverges from classical theories and conceives their institutionalization in a less strict way, as they lack formally strong structuration. The text shows the capacity of informal party agreements to allow for relative degrees of institutionalization of a leadership that is originally disjointed. In this way the theoretical perspective indicates that party institutionalization does not necessarily implies an organizational development manifested in rutinized practices.