The formation of cellulases by Trichoderma viride in a medium containing cellulose as a sole source of carbon depends on the oxygen transfer rate (OSR); the OSR, on the other hand, depends on the concentration of cellulose in the medium because the concentration of cellulose strongly affects the viscosity of the medium. In the work presented here, the dependence has been determined for the oxygen transfer rate on geometric relations and viscosity in cellulose-containing media during cultivation in shaken flasks, and the oxygen transfer rate on N(Re') N(G') and N(a) during cultivation in a laboratory fermentor of 3000-mL volume. Two cellulosic materials have been compared with a different effect on viscosity: Microcrystalline beach cellulose and fibrous cellulose. It has been found that, in an applicable range of concentration, microcrystalline cellulose does not affect the oxygen transfer rate (at concentrations up to 3%). Fibrous cellulose increases the OSR during cultivation in shake flasks but decreases it during civilization in fermentors. On the basis of these results, the optimization has been carried out on the cultivation conditions in fermentors.