Examining the mega-city-region hypothesis: evidence from the Paris city-region/Bassin parisien, Regional Studies. Four dimensions of the concept of polycentricity (morphological, functional, relational, and political polycentricity) are discussed based on the study of the Paris city-region/Bassin parisien system. The concentration of 'Abstract Production' workers in the Paris agglomeration and the fairly concentrated geography of firms' information flows (as measured by their telephone calls) prevent the formation of an enlarged polycentric mega-city-region. Three explanations are proposed based on the strategies of real estate developers, information-intensive firms and policy-makers. In conclusion, the paper tries to evaluate in the Paris city-region and French context whether a limited polycentricity at Bassin parisien level is prejudicial at different policy-relevant scales.