The application of anaerobic submerged membrane bioreactors was studied for the treatment of wastewaters containing suspended solids. A mesophilic and a thermophilic reactors were operated with a synthetic wastewater. The thermophilic reactor achieved higher volumetric loading rates than the mesophilic reactor, reaching 14g COD/Ld (0.47g COD/g VSSd). The mesophilic reactor showed signs of overload, when reaching a volumetric loading rate of 10g COD/Ld (0.32g COD/g VSSd). Cake formation was identified as the main factor governing applicable flux. Low levels of irreversible fouling were observed in both reactors. Low fluxes were attained and gas sparging was ineffective in increasing the critical flux.