Alginate production by Azotobacter vinelandii in chemostat cultures was evaluated at different dilution rates (D) and inlet sucrose concentrations of 5 and 20gl-1. At the low inlet sucrose concentration, the molecular weight of alginate increased from 800 to 1800kDa when D increased from 0.05 to 0.10h-1, whereas the opposite trend was observed with the high inlet sucrose concentration. This behaviour can be explained by changes in specific sucrose uptake rate. Thus, a decrease in alginate molecular weight was dependent on the specific sucrose uptake rate when this rate was higher than 0.42gg-1h-1. The manipulation of the D can be used to select the molecular weight of alginate in continuous culture.