The effect of oscillating dissolved oxygen tension (DOT) on the metabolism of an exopolysaccharide-producing bacteria (Azotobacter vinelandii) was investigated, particularly on the mean molecular weight (MMW) of the alginate produced. Sinusoidal DOT oscillations were attained by manipulating the oxygen and nitrogen partial pressures at the inlet of a 1.0 L working volume bioreactor. Periods of 1200, 2400, and 4000 s and average amplitudes between 1.0% and 2.2% DOT, with an oscillation axis fixed at 3% DOT, were tested. A culture carried out at constant 3% DOT was used as comparison. The average wave amplitude had an important effect on the maximum mean molecular weight (MMW(max)) of the alginate produced. The higher the amplitude, the lower the MMW(max). As the average wave amplitudes decreased from 2.2% to 1.0%, the MMW(max) increased from 64 to 240 KDa, respectively. Furthermore, at 3% constant DOT (0.0% of amplitude), a MMW(max) of 350 KDa was obtained. No important effect of the oscillating DOT on kinetics of biomass growth, alginate production, and sucrose consumption was observed, compared with constant DOT. The findings of this study point out that accurate DOT control is crucial if a particular molecular weight species of alginate needs to be produced, particularly in large fermentors, where bacteria are exposed to an oscillatory environment as a result of DOT gradients caused by the high viscosity of the broth and insufficient mixing.