Wood-boring animals belonging to at least two different metazoan phyla have been shown to possess cellulytic enzyme systems (1, 2). Marine borers of the genus Teredo have been shown (3) to contain up to 50 per cent glycogen on a dry weight basis. This observation, combined with the fact that the animal spends its entire adult life span embedded in wood, prompted an investigation of cellulase activity in this form. It is the dual purpose of this communication briefly to report the existence of such an enzyme system and to describe some of the observed conditions of its activity in vitro.