The influence of hydrodynamic conditions on the start-up phase of an inverse turbulent bed bioreactor was investigated. Two identical reactors, differing only by the gas velocity ensuring the carrier fluidization and generating the main hydrodynamic strengths (attrition), were monitored. Regarding the first 96 days, the experiments showed better performances for the reactor having the lowest hydrodynamic strengths. These results were correlated to lower attached biomass and higher biofilm specific removal rate for the reactor subjected to strong hydrodynamic strengths. Once the start-up was completed and the reactor stabilized at an organic loading rate of 6g(COD)L(-1)d(-1), the same hydrodynamic strengths were applied (gas velocity was equalized). The results proved that hydrodynamic conditions make it possible to control the biofilm characteristics. Consequently, it should be recommended to apply minimal strengths to enhance the biofilm growth during the early start-up phase, and then to increase them to control biofilm thickness and to optimize the reactor performances.