The effect of biomass concentration on the formation of Aspergillus oryzae alpha -amylase during submerged cultivation with A. oryzae and recombinant A. nidulans strains has been investigated. It was found that the specific rate of alpha -amylase formation in chemostats decreased significantly with increasing biomass concentration in the range of approx. 2-12 g dry weight kg(-1). When using a recombinant A. nidulans strain in which the gene responsible for carbon catabolite repression of the A. oryzae alpha -amylase gene (creA) was deleted, no significant decrease in the specific rate of alpha -amylase formation was observed. On the basis of the experimental results, it is suggested that the low value of the specific alpha -amylase productivity observed at high biomass concentration is caused by slow mixing of the concentrated feed solution in the viscous fermentation medium.