Two bioreactor continuous cultures, at anaerobic and aerobic conditions, were carried out using a recombinant Saccharomyces cerevisiae strain that over-expresses the homologous gene EXG1. This recombinant system was used to study the effect of dissolved oxygen concentration on plasmid stability and gene over-expression. Bioreactor cultures were operated at two dilution rates (0.14 and 0.03 h-1) to investigate the effect of other process parameters on EXG1 expression. Both cultures suffered severe plasmid instability during the first 16 generations. Segregational plasmid loss rate for the aerobic culture was two-fold that of the anaerobic operation. In spite of this fact, exo-β-glucanase activity at aerobic conditions was 12-fold that of the anaerobic culture. This maximal activity (30 U ml-1) was attained at the lowest dilution rate when biomass reached its greatest value and glucose concentration was zero. © Springer-Verlag 2004.