The continuous treatment of fish meal processing wastewaters, after a centrifugation step to remove solids, was carried out in a mesophilic Upflow Anaerobic Filter (UAF). After an initial period where the organic loading rate (OLR) and salinity of the influent was increased stepwise, the maximum applied OLR ranged from 5 kg COD/m3·day with a salinity content around 7.5 g Cl-/l. Total and free ammonia concentrations ranged from 2.0 g N-TA/l and 0.3 g N-FA/l, respectively. Recycling ratio was a key factor for the performance of the UAF since the change from 1:10 to 1:5 (F:R) caused the appearance of high concentrations of VFA, ammonia (up to 6.5 g N-TA/l and 1.3 g N-FA/l) and a sharp accumulation of VSS in the effluent. These effects were probably caused by the limitation of mass transfer rate and by a certain acidification in the bottom part of the reactor caused by the local increase of substrate concentration, this causing cell lysis and the los of part of the biomass. However, even under these conditions COD removal efficiency was always higher than 80%. The biomass developed in the reactor was mainly immobilised, either by attachment on the support or by entrapping, leaving its specific methanogenic activity higher than 1.0 g CH4 - COD/g VSS·day at the end of the operation.