A kinetic study was carried out on the aerobic treatment of olive-mill wastewater by fermentation with Aspergillus terreus. The bioreactor used was batch fed at several concentrations. The aerobic treatment process followed a Quiroga and Sales model, which is represented by a second-grade polynomial from which the kinetic constants; maximum substrate concentration (h), maximum specific growth rate (P) and organic matter present in the medium (q) were calculated by using a non-linear regression. The mean values for chemical oxygen demand (COD) elimination velocity were 126.3 mg l(-1) h(-1) in the first 24 h and 77.3 mg l(-1) h(-1) at 72 h, An increase in airflow allowed higher degradation percentages in less time (COD: 65.77% and BOD: 85.41%). The kinetic parameters 'q = S', corresponds to the organic matter present in the medium, which cannot be metabolized by the microorganisms under operating conditions. The value of P = mu(max) was influenced by the transfer of pure oxygen, the value of theta(c)(M) (minimum cellular retention time) decreased from 22.42 h for air with a porous plate to 8.31 h for pure oxygen. (C) 2002 Elsevier Science Ltd. All rights reserved.