The timing of combined additions of oxygen and assimilable nitrogen was optimized. Five mg/l oxygen and 300 mg/l (NH4)(2)HPO4 were added (i) before inoculation, (ii) at the end of the cell growth phase or (iii) at the halfway point of fermentation. There were marked differences between the nine combinations. In all cases, adding oxygen at the end of cell growth and nitrogen at the halfway point of fermentation appeared to be the most effective combination for completion of the fermentation whereas initial additions always had little favorable effect on this completion. The poor effect of initial oxygenation was partially explained by the large fraction of antimycin-sensitive oxygen consumption at the beginning of fermentation. The technological usefulness of optimized combined additions was shown by testing them during 10 different sluggish fermentations : the mean duration was almost 50% of the mean duration for control fermentations.