Historically, high performance anaerobic wastewater treatment processes have been developed by maximizing the retention of microorganisms either by fixing them on porous material or by favoring the growth of easy-settling microbial granules. While numerous studies have been directed towards the comprehension of the various processes involved in the fixation of microorganisms or the formation of microbial granules, much less work has been devoted to the improvement of solid-liquid separation techniques. The recent development of a new generation of ultra-filtration membranes more productive and less expensive has however prompted the emergence of a new concept in the wastewater treatment technology : the membrane bioreactor. The study is aimed at the determination of the optimal operating condition of an anaerobic membrane bioreactor. The influence of biological and physico-chemical parameters on the overall performance was evaluated. It was found that the performance of the biological component of the system depended mainly on the mass loading while that of the separative part was influenced only by physical parameters such as pressure, cross-flow velocity and suspended solids concentration. A filtration testing procedure was developed and several relationships were established between the various operating parameters of a membrane bioreactor.