The! extraction of bacterial alginate from batch fermentation broths of Azotobacter vinelandii NCIMB 9068 was performed using crossflow filtration through ceramic membrane modules of nominal pore size (PS) ranging between 0.20 and 1.4 mum. All tested membranes except 1.4-mum-PS one retained bacteria efficiently. Whatever the membrane PS, between 70 and 80% of the polysaccharide produced could be recovered by the filtration process. The permeation percentage increased with the membrane PS while membrane fouling was observed with all membranes but decreased with the PS. From these results, the 0.80-mum-PS-membrane module was selected for continuous alginate production in a membrane bioreactor with on-line cell recycling. Alginate production was efficient for about 70 h, after which membrane fouling occurred. A total of 7.55 g of polysaccharide was recovered from the permeate, the production rate (0.09 g h(-1)) and yield (0.21 g alginate g(-1) sucrose) being significantly higher than in the batch process. A decrease in alginate (average) molecular weight and an increase in alginate polydispersity were observed as incubation proceeded, probably due to enzymic degradation of the polysaccharide. (C) 2002 Elsevier Science Ltd. All rights reserved.