AbstractAlthough air‐lift fermentors have been employed industrially and in the laboratory, little information has been published on the effects of design on performance. With respect to both liquid circulation and mass transfer, not only the actual rates but the efficiency in relation to power consumption is strongly influenced by lift height and diameter, submergence ratio, and air‐flow rate. Relatively wide tubes, operating at high submergence ratios and rather low air‐flow rates favor high efficiency. Since these conditions lead to rather low absolute values of circulation and mass‐transfer rates, the microbial population which can be supported will also be rather small. Mass transfer can be increased by the insertion of an orifice some distance; above the air‐inlet point and by suitable arrangement of the discharge from the air lift into the headspace of the