The complex symbiotic microbiota in the rumen is responsible for the enzymatic breakdown of plant fibres, an ability the ruminant host animal lacks. This microbiota is highly responsive to changes in diet, age, antibiotic use, and the health of the host animal, and varies according to geographical location, season, and feeding regimen. Fibrobacter succinogenes and Ruminococcus flavefaciens are considered to be the predominant cellulolytic bacteria present in the rumen (Krause et al., 2003). Ruminal pH fluctuates considerably in a 24h period and is influenced by the intake of fermentable carbohydrate, the inherent capacity of the animal to produce saliva, and rates of utiliszation and absorption of acids. In beef cattle fed high-concentrate diets, the ability of the animal to buffer the rumen is limited by inadequate salivary secretion. The objective of the present experiment was to investigate the effect of fluctuation in ruminal pH on the population of F. succinogenes in rumen fluid.