A jet-loop activated sludge reactor (JLR) of 15 dm(3) working volume was used for the aerobic treatment of winery wastewater (WW). The reactor was operated continuously for more than 12 months using WW from different wineries and collected in different periods of the year with CODs that ranged between 0.8 and 12.8 kg m(-3). Loading rates were from 0.4 to 5.9 kg-COD m(-3) day(-1) with hydraulic retention times that varied from 2.1 to 4.4 days. The system reacted well to sudden variations of loading: the COD removal efficiency was always higher than 90% with an organic load of the final effluents that ranged between 0.11 and 0.3 kg-COD m(-3). After 185 days of continuous operation, the residential microbial population, of either the mixed liquor or the biofilm formed on the reactor's inner wall, was studied and characterized at the genus level. Most isolates belong to the genus Pseudomonas and the yeast Saccharomyces cerevisiae. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.