AbstractDissolved oxygen measurements were made in pilot (20 and 250 l.) and production scale (15,000 l.) novobiocin fermentations. Bulk mixing was found to be incomplete in pilot tanks with turbine impellers ofD/T= 0.40 (whereDis impeller diameter, andTis tank diameter) but appeared homogeneous with impellers ofD/T= 0.69. In the former case, the respiration rate was presumably limited by insufficient oxygen supply in areas of poor bulk mixing, whereas, in the latter case, the major resistance was between the bulk of the liquid and the cell (intraclump resistance). Higher agitator speeds decreased the gas–liquid resistance proportionally more than they reduced the liquid‐cell resistance. In production fermentors, dissolved oxygen measurements indicated that bulk mixing was complete with each of the three impeller sizes tested (D/T= 0.28, 0.33, and 0.43), but that the respiration rate was again limited, mainly by a resistance between the bulk of the liquid and the c