The effect of carbon dioxide on the production and quality of alginate by Azotobacter vinelandii was evaluated in batch cultures conducted in a 1 L bioreactor under constant dissolved oxygen tension of 3%, using different levels of CO2 (0-25% v/v) in the income gas stream. The effect of CO, on the process was assessed in terms of biomass growth, product formation, and substrate consumption. The impact of CO, addition on the polymer molecular weight was also quantified. Biomass growth and alginate yield was first inhibited (4-8% CO,) and then stimulated (13% CO2). For 25% CO,, bacterial growth and alginate production were totally inhibited. For low added CO2 (<4%) the mean molecular weight at the end of the culture dropped dramatically. This drop was not observed when 8% or 13% CO, was added. The results suggest that high CO, concentrations inhibit the synthesis or activity of polymer-degrading enzymes (alginate- lyases).