The evolution of biomass contained in a pilot-scale digester treating wastewater from a sea-food processing factory (15-45 g Chemical Oxygen Demand (COD) dm(-3) and high salinity) was studied for 2 years. During this period, different effluents have been treated and several operational conditions were followed. Laboratory-scale experiments were carried out to determine the sludge methanogenic activity and the salinity adaptation of the biomass which developed in the digester. During the different periods, sludge concentration remained between 10 and 12 g Volatile Suspended Solids (VSS) dm(-3), a value that seems to be characteristic for this reactor. A global Organic Loading Rate (OLR) balance showed no significant change of biomass concentration inside the reactor, although a quite important growth of biomass (11.5% of OLR fed) was observed. Methanogenic activity assays indicated a sludge with a good activity (0.5-0.75 g COD g(-1) VSS day(-1)) in a saline medium could be obtained from a low activity sludge (0.047 g COD g(-1) VSS day(-1)). Toxicity assays showed the importance of antagonistic effects of other cations on the toxicity exerted by sodium.