Résumé : Metropolises are the focal places from which world economic exchanges are structured and where international actors are located. In regard to the objective of reinforcing their position in the urban hierarchy, air services represent an essential issue for metropolises. This paper aims at comparing and understanding the differentiation of air services in Brussels and Zurich, considered from a quantitative (volume of supply) and geographical (diversity of connections) point of view. The analysis is conducted by taking into account the characteristics of both metropolises and the strategies followed by the airline companies and airport managers. It appears that Brussels owes a good proportion of its air services to the presence of international public institutions whereas Zurich is "over-served" thanks to the hub of Swiss International Air Lines, which is based on financial functions of world importance.