The location of cellulase inC.fulvusdepends on the carbon source for growth and the age of the culture. When cells were grown on glucose or cellobiose all CMC-hydrolyzing enzyme was cell-bound but only part of the activity was located on the cell surface. Treatment of cells with EDTA, lysozyme, and detergents and subsequent fractionation experiments showed that cellulase was also located in the periplasm and bound to a membrane fraction.Growth on cellulose gave cell-free cellulase active against CMC. The enzyme was repressed by glucose but formed at a constant differential rate on cellobiose and amylose. This rate was 8–10 times lower than on cellulose and possible reasons for this are discussed.