The influence of oxygen transfer rate (OTR) on the molecular mass of alginate was studied. In batch cultures without dissolved oxygen tension (DOT) control and at different agitation rates, the DOT was nearly zero and the OTR was constant during biomass growth, hence the cultures were oxygen-limited. The OTR reached different maximum levels (OTRmax) and enabled to establish various relative respiration rates. Overall, the findings showed that OTR influences alginate molecular mass. The mean molecular mass (MMM) of the alginate increased as OTRmax decreased. The molecular mass obtained at 3.0 mmol l-1 h-1 was 7.0 times higher (1,560 kDa) than at 9.0 mmol l-1 h-1 (220 kDa). An increase in molecular mass can be a bacterial response to adverse nutritional conditions such as oxygen limitation.