The effect of different aeration conditions during the culture of Azotobacter vinelandii on the production and molecular mass of alginate was evaluated in shake flasks. In baffled flasks, the bacteria grew faster and produced less alginate (1.5 g/l) than in conventional (unbaffled) flasks (4.5 g/l). The viscosity of the culture broth was also influenced by the type of flask. Higher final viscosities were attained in unbaffled flasks [520 cP (520 mPa s)] as compared to baffled flasks (30 cP). This latter phenomenon was closely related to the changes in the molecular mass distribution. In either cases, the mean molecular mass increased with culture age; however, at the end of the fermentation, the mean molecular mass of the alginate obtained in unbaffled flasks was fivefold higher than that obtained in baffled flasks. As the culture proceeded, the cells of Azotobacter grown in unbaffled flasks increased in diameter, whereas those cultured in baffled flasks decreased in size.