Submerged mycelial growth of Aspergillus kawachii IFO4308 in solid-state culture (SSC) was studied. From the result of Northern blot analysis, acid-stable alpha-amylase was found to be produced mainly by the submerged mycelia rather than the aerial mycelia. The submerged mycelia showed better growth in SSC using rice as the solid substrate (koji) than in agar plate culture in spite of low concentrations of dissolved oxygen in koji. Good growth in SSC suggested the existence of an effective oxygen transfer mechanism in koji which governed the mycelial growth. When koji was submerged in water, small bubbles were generated. This phenomenon indicated the formation of vacant spaces in koji during SSC. The submerged mycelia showed better growth in the koji having a larger number of vacant spaces. Considering these facts it was concluded that the vacant spaces participate in effecting an oxygen transfer mechanism in koji as air vents because the diffusivity of oxygen in an air is larger than in koji itself.