Competition for adhesion to cellulose among the three main ruminal cellulolytic bacterial species was studied using differential radiolabeling (14C/3H) of cells. When added simultaneously to cellulose, Ruminococcus flavefaciens FD1 and Fibrobacter succinogenes S85 showed some competition; however, both species were surpassed competitively by Ruminococcus albus 20. When R. flavefaciens FD1 and F. succinogenes S85 were already adherent, R. albus 20 adhesion occurred without inhibition but involved R. flavefaciens FD1 detachment.