Abstract 1. 1. Seventy-four species of animals were investigated to determine whether they have cellulase activity in their digestive glands. Cellulase activity, detected by noting the reduction in viscosity of a solution of sodium carboxymethylcellulose, was found in a number of annelids, molluscs and crustaceans. In contrast to these findings, all vertebrates and related groups such as ascidians and holothurians which were examined were shown to be lacking in cellulase activity. 2. 2. A proposal is put forward that the distribution of cellulase within the animal kingdom is more closely correlated with animal phylogenetic relationships than with differences and similarities in their feeding habits.