Résumé : Since 1989 and the regionalization of public transports in Belgium, the Société des transports intercommunaux de Bruxelles (STIB), i.e. the public operator of urban transport in Brussels, has undergone many changes, both structural and organizational. Meanwhile, the company has managed to improve its financial health in accordance with the objectives set by the regulatory transport authority, the Region of Brussels-Capital. The analysis of the system of actors shows that, in a principal-agent relationship characterized by high information asymmetry, the convergence of interests between the supervisory authority and the public company is neither natural nor automatic: it is built. This construction takes place at several levels and materializes incrementally, particularly through the management contract. In this regulatory system, the process organizing the dialogue between the company and the authority produces the desired beneficial effects, more than the legal value of the contract does. The interdependence between the actions of the Region and those of the STIB implies practices that can be described as a partnership. Finally, the analysis shows that the contractualization is not inconsistent with an evolution of the roles of each actor.