Using the technique of incorporation of H-3-thymidine into DNA, we describe colonization properties of an anaerobic, mesophilic, cellulolytic Clostridium, strain C401. This method took into account both bacteria which adhered and those which did not adhere to the substrate. The observed generation time (7.5 h) was faster than that detected (26 h) with other methods. Under the conditions used, the end of growth was characterized by a sharp release of biomass. A depletion in the supply of carbon source and therefore, of the adhesion site, was responsible for this release.