Alginate production by Azotobacter vinelandii growing in chemostat cultures was evaluated under different O-2 transfer rates (OTR). As a result of modifying the culture's agitation rate from 300 to 500 rpm, the OTR increased from 9 to 15.1 mmol l(-1) h(-1) and a slight variation in the alginate production (1.7-2.2 g l(-1)) was observed. At a constant growth rate (0.1 h(-1)), the mean molecular mass of the alginate was strongly influenced by changes in the OTR, varying from 860 to 1,690 kDa. These results support a possible relationship between alginate polymerization-depolymerization process and the O-2 uptake rate.