A spontaneous adhesion-defective mutant (mutant D5) of Ruminococcus albus strain 20 was isolated and compared to the parent to investigate the impact of the mutation on cellulolysis and to identify the adhesion mechanism of R. albus. The comparison of kinetics of cellulose degradation by strain 20 and mutant D5 showed that the mutation delayed and reduced bacterial growth on cellulose and cellulose degradation. These results were partly explained by a twofold lower cellulase activity in the mutant than in the parent. The glycocalyx of strain 20, observed by transmission electron microscopy, was large and homogenous, and linked cells to cellulose. The mutant glycocalyx was aggregated at its periphery and cells attached loosely to cellulose. A glycoprotein of 25 kDa (GP25), present in the membrane fraction and the extracellular medium of strain 20, was not detected in the same fractions of mutant D5. Though glycoprotein GP25 did not bind to cellulose, it may be involved in adhesion as an intermediate component. Different cell-surface features of mutant D5 (cellulases, glycoprotein GP25, glycocalyx) were thus affected, any or all of which may be involved in its adhesion-defective phenotype. These results suggest that adhesion and cellulolysis are linked and that adhesion is a multifactorial phenomenon that involves at least the extracellular glycocalyx.