The location and formation of cellulases and β‐glucosidase in the fungusTrichoderma viridewere studied on different substrates. The cellulases were found to be cell‐bound during active growth on cellulose, CMC, and cellobiose. On CMC, much CM‐cellulase was found cell‐free but sonication released cellulase from the washed mycelium. Analysis of the carbohydrates of the mycelial cell wall after hydrolysis revealed glucose, mannose, and galactose—the same carbohydrates as reported to be present in purified cellulase from the same organism. Glucose repressed the formation of both CM‐cellulase and Avicelase and cellobiose apparently the formation of Avicelase. Relatively little CM‐cellulase was formed on cellobiose but a straight line was obtained when a differential plot of CM‐cellulaseversus