The relation between dissolved oxygen (DO) and the ajmalicine production rate of Catharanthus roseus was investigated in 15-L tank reactors at constant stirrer speed and gas flow rate. Below a DO concentration of 29% of air saturation the ajmalicine production rate was less than 0.06 mu mol/g/d. Above a DO of 43% the ajmalicine production rate was constant at 0.21 mu mol/g/d. Between a DO of 29% and 43% there was a strong relation between the ajmalicine production rate and the DO concentration. After a period of at least 12 days at DO less than or equal to 29% the culture lacked the ability to adapt to a DO greater than or equal to 57%. A kinetic equation is proposed for the relation between DO and the specific ajmalicine production rate.