Cellulose is the major polysaccharide component of plant cell walls and is the most abundant organic compound on the planet. A number of bacterial and fungal organisms can use cellulose as a food source, possessing cellulases (cellobiohydrolases and endoglucanases) that can catalyse the hydrolysis of the beta-(1,4) glycosidic bonds. They can be classified into seven distinct families. The three-dimensional structures of members of two of these families are known. Here we report the structure of a third cellulase, endoglucanase V, whose sequence is not represented in any of the above families. The enzyme is structurally distinct from the previously determined cellulases but is similar to a recently characterized plant defence protein. The active site region resembles that of lysozyme, despite the lack of structural similarity between these two enzymes.