Eleven strains of Aspergillus oryzae were screened for production of β-galactosidase on media based on lactose and wheat bran. All the strains produced highest levels of activity on the wheat bran medium. Production by the best strain, A. oryzae VTT-D-88353 (ATCC 20423) was considerably better than that obtained with A. niger VTT-D-80144, previously reported as an efficient producer of β-galactosidase, in both shake flask and laboratory bioreactor cultivations. Production on a lactose-based medium in the bioreactor was better in fed-batch than in simple batch cultivation, but considerably poorer than on wheat bran. The role of the organic nitrogen source was also of major importance for β-galactosidase production by the A. oryzae strain. The cultivation time required for maximal production by A. oryzae was longer than that required by A. niger and could not be reduced by the use of surface active agents. In lactose hydrolysis experiments the enzyme of A. oryzae was somewhat more efficient that that of A. niger at near-neutral pH values at 30°C, but at 50°C the A. niger enzyme was more effective at all pH values between 5.0 and 7.0.