BACKGROUND: Anaerobic treatment of saline wastewaters may be hindered by problems related with biomass retention, since at high salt concentrations formation of biofilms and granules may not proceed well. This research studied the use of anaerobic membrane bioreactors (AnMBR) as a way to promote complete biomass retention. A lab scale AnMBR fitted with a ceramic tubular membrane was operated for 2 years. RESULTS: Results showed that enhanced biomass retention produces conditions enabling anaerobic treatment of saline wastewaters. Despite the high resulting sludge retention time, no accumulation of a high proportion of dead cells was observed. Protein degradation and not methanogenesis was shown to be the rate limiting step for organic matter degradation, a fact that is relevant for protein-containing wastewaters such as those from seafood processing industries. Only low levels of flux could be applied, in the region of 5 L m-2 h-1 due to reversible cake formation promoted by single cell growth. CONCLUSION: Biomass retention provided by membrane filtration promotes conditions suitable for efficient treatment of saline wastewaters. However, operation may be restricted to low values of flux due to biomass development as single cells.