The feasibility of obtaining useful interspecific hybrids with high cellulolytic activities by breeding was investigated by a protoplast fusion technique using the citric acid hyper-producer, Aspergillus kawachii, and the cellulase hyper-producer, Aspergillus niger. Interspecific hybridization between two auxotrophic mutants of Aspergillus kawachii (his) and A. niger (met) was achieved by protoplast fusion in the presence of 35% polyethylene glycol as fusogen. Stable diploid strains were obtained from a heterokaryon strain by treatment with 0.1% d-camphor. The selected strains were specified by their conidial size and DNA content as well as the conidial germination. One of them showed high cellulase productivity besides the same level of amylase production and sporulation as that of A. kawachii, a higher level of citric acid and the lowering of mash viscosity than that of A. kawachii. From these results it is suggested that this diploid strain would be very useful for the degradation of cellulosic substances in the mash from shochu fermentation, consequently for its ready disposal thereafter.