SUMMARY: The effect of size of inoculum on growth and carbon metabolism of Aspergillus oryzae and several other Aspergillus species was studied. During most of the development a culture from a small inoculum, as compared to a large-inoculum culture, had a smaller specific rate of respiration, formed more ethanol and acids per unit weight of mycelium, and metabolized more carbohydrate to produce a given amount of growth. It seems that this inefficient utilization of carbon source is due not only to the production of a smaller amount of energy but also to an inefficient utilization of the released energy. The higher degree of inefficiency appears to be the result of the aging process which a small-inoculum culture undergoes during its development to the stage of growth of a young large-inoculum culture; the latter also shows a decrease in efficiency during cultivation.