Addition of a hydrocarbon to fermentation medium has been recognized as a means of enhancing oxygen transfer. However, the mechanism involved has not been clarified. We investigated the glucose oxidase fermentation using Aspergillus niger in the presence of n-dodecane, n-hexadecane and soybean oil. The hydrocarbons had positive influences on the formation of both intra- and extracellular glucose oxidases. Moreover, the fermentation time needed to achieve the maximum enzyme yield was reduced. The increase in the yield of glucose oxidase was due to an increase in the efficiency of enzyme synthesis in the cells for cases of n-dodecane or n-hexadecane, while it was due to an increase in cell concentration for soybean oil. The increase in the level of extracellular glucose oxidase arose from increased enzyme secretion. However, it was interesting to find that the hydrocarbons delayed the cell growth as well as the production of glucose oxidase. This fact implies that the hydrocarbons alone do not improve the oxygen transfer. An explanation was made by considering the timing of feeding of the antifoaming agent KM-70 into the fermentation broth. The improved effects of the hydrocarbons on the fermentation are due to their interaction with the antifoaming agent. Accordingly, the timing of addition of the hydrocarbons to the culture is an important determinant for the fermentation. Apparently, the optimal timing is shortly after the start of feeding of the antifoaming agent.