The fermentation of ryegrass straw, which is a semisolid substrate, is studied. In a semisolid fermentation, a thin water film is formed surrounding the insoluble substrate, which makes the diffusion of oxygen into the water faster than in liquid fermentation. Therefore, the oxygen requirement level for semisolid fermentation is expected to be less than for liquid fermentation. This paper studies the oxygen demand level of C. utilis grown on semisolid straw by measuring the growth rate of the organism under various levels of oxygen supply.