SUMMARYThe growth behavior ofChaetomium cellulolyticumin cellulose medium indicated that long fibers of cellulose (Solka-Floc S.W. 40) are broken into shorter ones during the early phases of fermentation followed by longitudinal splitting of short fibers into fibrils and ultimately almost complete utilization of the substrate.Chaetomium cellulolyticumwas found to be a fast-growing organism on 1% cellulose with a protein synthesis rate of 0.09 h-1. The rate of protein synthesis increased more than threefold (0.3 h-1) when 1% more substrate was added immediately after the complete conversion of the first batch of substrate. The growth behavior ofC. cellulolyticumon cellulose indicated that it might be the most suitable organism for cyclic-batch or continuous fermentation of cellulose for microbial protein production. NaOH utilization, to keep the level ofpH at 5.0 during fermentation, correlated positively with the synthesis of protein. The rate of NaOH utilization during fermentation could be used as an indication of growth of or protein synthesis by an organism when other methods cannot be used immediately.