The cellulolytic enzyme complex of Clostridium thermocellum is very large.

The cellulolytic enzyme system bound to cellulose during the early stages of growth of C. thermocellum on this substrate was resolved into two major complexes. These complexes, as viewed by electron microscopy, are spherical particles with diameters of 210 A and 610 A and calculated molecular weights of 4.2 million and 102 million daltons, respectively.