The effects of oxygen transfer on the beta-lactamase production by Bacillus licheniformis were investigated in a glucose-based defined medium. The experiments were conducted in 3.0 dm(3) batch bioreactor systems at three different air inlet (Q(O)/V-R = 0.2, 0.5 and 1.0 vvm) and agitation rates (N = 250, 500 and 750 min(-1)). During the fermentation, the concentrations of the cell, glucose, by-products, ie organic and amino acids, oxygen transfer coefficients (K(L)alpha), yield coefficients, specific rates and oxygen uptake rates (OUR) were determined, in addition to beta-lactamase activities. The highest beta-lactamase activity was obtained at Q(O)/V-R = 0.5 vvm and N = 500 min-1 and at Q(O)/V-R = 0.2 vvm and N = 500 min-1 conditions, as ca A = 90 U cm(-3). The highest cell concentration was obtained as C-X = 0.67 kg m(-3) at Q(O)/V-R = 0.5 vvm and N = 750 min(-1) and at Q(O)/V-R = 0.2 vvm and N = 750 min(-1) conditions. The values of K(L)a increased with increasing agitation and aeration rates and varied between 0.007 and 0.044 s(-1), and the OUR varied between 0.4 and 1.6 mol m(-3) s(-1). With increasing Q(O)/V-R and/or N, the Damkohler number (ie the oxygen transfer limitation) decreased owing to the increase in mass transfer coefficients (K(L)a). The highest instantaneous yield of cell on substrate (Y-X/S) and yield of cell on oxygen (Y-X/O) values were respectively obtained at 0.5 vvm and 500 min-1 conditions at t = 2 h as Y-X/S = 0.72 kg kg 1 and Y-X/O = 1.49 kg kg(-1). The highest instantaneous yield of substrate on oxygen (Y-S/O) was obtained at 0.5 vvm and 750 min(-1) conditions at t = 20 h as Y-S/O = 8.07 kg kg(-1). (c) 2005 Society of Chemical Industry.