Growth and alginate production by Azotobacter vinelandii DSM576 as a function of initial ammonium sulphate concentration (0.451.05 g l1) and agitation speed (300700 rpm) were studied in batch fermentations at controlled pH. The time course of growth, alginate production and substrate consumption and the effect of nitrogen concentration and agitation speed on kinetic parameters and on maximum alginate molecular weight (MW) was modelled using empirical equations. The kinetics of growth, alginate production and polymerization were deeply affected by agitation speed and, to a lesser extent, by inorganic nitrogen concentration. Average and maximum specific growth rate and maximum alginate MW all increased with agitation speed, and were higher at intermediate ammonium sulphate concentration. Maximum alginate MW (250,000) was obtained at high agitation speed (600700 rpm) and alginate depolymerization was limited or did not occur at all when the agitation speed was higher than 500 rpm, while at 400 rpm depolymerization significantly reduced the alginate. However, alginate yield was negatively affected by increasing agitation speed. A good compromise between alginate yield (2 g l1) and quality (MW250,000) was obtained with agitation speed of 500600 rpm and 0.750.90 g l1 of ammonium sulphate. Journal of Industrial Microbiology Biotechnology (2000) 25, 242248.