A membrane bioreactor which combines a continuous fermentor and a cross-flow microfiltration module enabling continuous cell recycling and removal of the fermentation products freed of microbial cells was tested for the production of alginate by Azotobacter vinelandii. The molecular weight characteristics of the polysaccharide produced were investigated. The macromolecular characteristics of alginate recovered by first filtering batch fermentation broths through different microporous ceramic membranes with pore sizes varying from 0.45 to 1.4 microm seemed to be independent of the MPS. The 0.8-microm membrane pore size (MPS) was selected for continuous alginate production in the membrane bioreactor. A mean yield value of 0.21 g alginate g(-1) saccharose was obtained and an increase in alginate polydispersity as incubation proceeded was observed, probably due to enzymatic degradation of the polysaccharide.