Oxygen transfer (OT) effects on benzaldehyde lyase (BAL) production by recombinant Escherichia coli carrying pUC18::bal gene were investigated on a defined medium with glucose and (NH4)(2)HPO4 as the sole carbon and nitrogen sources respectively, in 3.0 dm(3) bioreactor systems at six different conditions with the parameters, air inlet rate of Q(O)/V-R = 0.5 vvm, and agitation rates of N = 250, 375, 500, 625, 750 min(-1) and at Q(O)/V-R = 0.7 vvm, N = 750 min(-1). Cell concentration (C-X), BAL volumetric and specific activities, organic acid concentrations, yield values, maintenance coefficients, oxygen transfer coefficients (K(L)a) and oxygen uptake rates (OUR) were determined throughout the bioprocess. Increase in the oxygen transfer rate increased the cell formation up to 0.5 vvm, 500 min(-1); however, further increase inhibited cell formation. The highest BAL volumetric activity was obtained at 0.5 vvm, 500 min(-1) as 860 U cm(-3). KLa, and enhancement factor E(K(L)a/K(L)a(o)) increased with the cultivation time and agitation rate, their values varied between 0.008-0.046s(-1) and 1.00-3.07, respectively. OUR first increased, and then decreased with the cultivation time at all OT conditions, the highest OUR was observed at 0.5 vvm, 500 min(-1) as 3.6 mmol m(-3) s(-1) at t = 8 h; while the highest specific OUR was obtained at 0.5 vvm, 250 min(-1) as 0.74 kg kg(-1) h(-1) at t 1 h. With the increase in Q(O)/V-R and/or N, Damkohier number (Da), that is the OT limitation, decreased. The effectiveness factor values were close to 1.0 at medium and high OT conditions at the beginning of the processes as cells consumed oxygen with such a high rate that maximum possible oxygen utilization rate (oxygen demand) value was approached; thereafter, eta decreased with the increase in cultivation time and Da, and the decrease rate was higher at lower agitation rates due to the lower K(L)a. Oxygen consumption for maintenance (m(O)) was the highest at 0.5 vvm, 500 min(-1) and the value was 0.20 kg kg(-1) h(-1). (C) 2004 Elsevier Ltd. All rights reserved.