The oxygen transfer rate (OTR) was evaluated as a scale-up criterion for alginate production in 3- and 14-L stirred fermentors. Batch cultures were performed at different agitation rates (200, 300, and 600 rpm) and airflow rates (0.25, 0.5, and 1 vvm), resulting in different maximum OTR levels (OTR max). Although the two reactors had a similar OTRmax (19 mmol L-1 h-1) and produced the same alginate concentration (3.8 g L-1), during the cell growth period the maximum molecular weight of the alginate was 1,250 kDa in the 3-L stirred fermentor and 590 kDa in 14-L stirred fermentor. The results showed for the first time the evolution of the molecular weight of alginate and OTR profiles for two different scales of stirred fermentors. There was a different maximum specific oxygen uptake rate between the two fermenters, reaching 8.3 mmol g-1 h-1 in 3-L bioreactor and 10.6 mmol g-1 h-1 in 14-L bioreactor, which could explain the different molecular weights observed. These findings open the possibility of using qO2 instead of OTRmax as a scaling criterion to produce polymers with similar molecular weights during fermentation.