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Five experiments were performed to determine the importance of chitin and cellulose in the diet of juvenile C. sapidus. A compartmentalized recirculating system was established to provide optimal conditions, maintaining the animals with little mortality. The appropriate ration, compartment size, and an adequate baseline diet were established. We replaced 20% of a commercial diet with varying amounts of chitin and cellulose. Crabs fed the cellulose-containing diet had higher growth rates, conversion efficiencies, molt increments and frequencies than crabs fed the chitin-containing diet, but were equal to the control diet. We then assayed for chitinase and cellulase in gut tissues. Chitinase had lower specific activity (0.072 + 0.159 mU mg-1min-1) than cellulase (3.52 + 0.16 mU mg-1min-1) in the foregut and hepatopancreas. There was no effect of diet on specific activity. The results show juvenile C. sapidus is capable of utilizing cellulose, but not chitin, when delivered as 20% of a diet.