In vitro fermentation studies were conducted to evaluate effects of initial medium pH on the extent of attachment on pure cellulose by one of major ruminal cellulolytic bacteria: Fibrobacter succinogenes via competitive PCR. After incubation for 10 min, the numbers of F. succinogenes attached to cellulose were 10⁹/g dry matter (DM) of cellulose at all pH levels. Thereafter, the numbers of F. succinogenes attached to cellulose did not change to an appreciable extent until 48 h at higher initial pH. However, at low initial medium pH (5.8), the numbers of F. succinogenes attached to cellulose gradually decreased and drastically dropped at 48 h incubation (10⁵/g DM). This may be the result of higher detachment rate or lower growth when F. succinogenes was exposed to initial medium pH less than 6.0. Higher initial pH resulted in higher DM disappearance. However, changes in the rate of in vitro cellulose disappearance were not synchronized with changes in the number of attached F. succinogenes.