Biohydrogen was produced using a granular biomass reactor coupled to a submerged internal membrane. The reactor performance was evaluated under different organic loading rates (OLR) ranging from 5 to 60 g L-1 d(-1) and hydraulic retention times (HRT) from 5.5 to 1.25 h. The UASB reactor was operated at 35 degrees C and pH 4.5. It was observed that the membrane introduction to the reactor does not affect the granule size or integrity. The maximum hydrogen production rate was obtained at 30 g L-1 d(-1) and 4 h of HRT (475 +/- 15 mLH(2) L-1 h(-1)). A further increase of the OLR resulted in a lower hydrogen production due to a shift of the metabolism to solvent production. The use of membranes allowed the application of relatively low HRT; however, HRT lower than 2 h promoted the homoacetogenic metabolism, decreasing the hydrogen production. The results indicate that the membrane fouling is not only affected by the total EPS formed but also by the operational flux applied.