The distribution of cellulase in the class Bivalvia is studied with respect to phylogenetic position, feeding category, and food. Cellulase activity in nine species is measured turbidimetrically using reprecipitated cellulase substrate, colorimetrically using methyl cellulose substrate, and histochemically with methyl cellulose film. Cellulolytic activity in these animals generally corresponds to the level of cellulose in the food. Those animals feeding on particles with a high cellulose content have high cellulase levels, while those organisms feeding on epiorganisms, diatoms, or phytoplankton with low cellulose content have low cellulase levels. The Gastrotriteia may have characteristically higher endogenous cellulase levels than the other major groups and certain genera may have characteristically low cellulase activity.