The influence of the hydraulic retention time (HRT) on the start-up phase of a methanogenic inverse turbulent bed bioreactor was investigated. Two identical reactors were monitored, the only differing parameter being the HRT : one of the reactors was fed with a diluted wastewater at a constant HRT of one day, the organic loading rate (OLR) being increased by decreasing the substrate dilution ; the second reactor was fed at a constant influent concentration of 20 gCOD.L-1, the OLR being increased by decreasing the HRT from 40 to 1 day. After 45 days of start-up, both reactors were operated at an OLR of 20 gCOD.L-1d-1 and a HRT of 1 day. However, strong differences were observed on biofilm growth. In the reactor operated at a constant short HRT, biofilm concentration was 4.5 as high as in the reactor operated at an increasing HRT. This difference was attributed to the competition between planktonic and biofilm microorganisms in the latter reactor, whereas suspended biomass was quickly washed out in the former reactor because of the low HRT.