International audienceThe competition authority (ARCEP) authorized in 2011 the introduction of a fourth mobile operator (Free) alongside Orange, SFR and Bouygues. More competitors was expected to mean more competition, which was supposed to improve the efficiency of the sector. This choice was questioned by Deffains [2012] who showed that the introduction of Free would entail the destruction of 50.000 jobs. Conversely Thesmar and Landier [2012] expected from that measure the net creation of 16.000 to 30.000 jobs. These studies were focussed on the effect on employment. Here, we conduct a cost-benefit study that takes into account all the consequences of this measure to consumers, businesses and public finance. The arrival of Free has result...