A nitrifying activated sludge reactor fed with a high salinity medium was operated efficiently at ammonia loading rates between 1 and 4 g NH4+ -N l(-1) d(-1). The system became completely inefficient at inlet salt concentrations higher than 525 mM due to the mixed inhibition effect of salts and ammonia. The final product was mainly nitrate although dissolved oxygen limitations caused sporadic ammonia and nitrite accumulations. Specific nitrifying activity decreased due to the saline effect. A set of activity tests showed that in the continuous reactor non-adapted biomass is rather more sensitive than biomass to the saline effect. Physical properties of biomass in the reactor (sludge volumetric index and zone settling velocity) were not affected by the saline concentration, a biomass concentration of 20 gVSS l(-1) was achieved.