An industrial-type fermentation for the production of alphaamylase was studied at 3-litre scale, using a complex medium and Bacillus subtilis B. 20, a high-yielding strain. It was found that both the peak titre and the production rate continued to increase up to the highest mass-transfer rates used. Using an impeller of half the fermenter diameter at speeds around 14 Hz (840 r. p. m. ) and aeration at 0.57 v. v. m., a batch fermentation of 60-70 hours gave at least 9 u/ml (about 900 S. K. B. u/ml, measured at pH 6.0, or 36 000 Novo u/ml). Rapid production began around hour 10, when growth was about half complete. The volumetric production rate was almost constant for 40-50 hours, giving a constant specific productivity of about 0.75 x 10-11 u/cell/h after the completion of growth, until an abrupt final decline. Rapid synthesis coincided with growth and the rate was not directly related to population, the rates of growth and mass-transfer or any other measured variable. The termination of synthesis in simple batch operation was apparently due to carbohydrate depletion. Thus, feeding increased production by about 25%. Corrected for dilution, the production rate was unchanged so that the specific productivity fell in proportion to the increase in population during feeding. It is suggested that the rate of synthesis was controlled by a pool of a very stable regulator-inducer complex accumulated before exhaustion of the inducer in the early stages of the fermentation. There appears to be considerable potential for further development along the lines followed in this work. The limits of the stimulatory effects of mass transfer and feeding were not passed. Neither was there evidence of depletion of a pool of complexed regulator.