International audience The aim of this paper is to better identify the decisive factors of the daily mobility by bus in urban areas. We try in particular to study to what extent the quality of the transportation supply by bus determines its use. In a first step we develop an indicator which describes locally the quality of the transportation supply by bus. In a second step, original data concerning moving practices have been collected through a telephone survey and implemented in a Geographical Information System. In a third step, the comparison between bus supply and its use shows that, if the quality of the bus supply determines the use only at an aggregate level, it is on the other hand strongly related to the level of satisfaction for the use of bus. Rather than through a global estimation of the bus supply, it appears that the modal choice can only be explain through the comparison of the bus and car supply for a given set of journeys.