Airlift reactors have found potential application in biotechnology and chemical industries due to their simple construction and less performance costs compared with mechanically stirred tanks. However, an accurate description of their performance and "a priori" design are still difficult. This work deals with mathematical modelling of the fermentation process in an internal loop airlift reactor (IALR). To verify the simulation results, fermentation of gluconic acid by the strain Aspergillus niger has been chosen as a model system. Fermentation was carried out in three laboratory lALRs (each one with different scale of the reactor volume: 12, 40 and 200 litres). The parameters of the model, e.g. parameters of the kinetic model, mass transfer coefficients, circulation velocities and gas hold-ups were estimated by employing correlations available in the literature or independent experiments using model media. In the present paper the influence of different performance conditions (gas inlet flow rate, oxygen concentration in the feed gas, initial concentration of glucose) on the reactor behaviour is investigated. The results of simulations and experiments are in a sufficient agreement. The developed and tested model can be used as a very useful tool for process design, optimization and scale-up.