Rumen pH, together with the microbial population, nature of substrates, environmental factors such as temperature and existence of cations and soluble carbohydrates have been suggested as factors governing bacterial attachment to ingested feed particles (Miron et al., 2001). Ruminal pH is one of the most important of these factors, because fibrolytic bacteria are very sensitive to pH change. Fibre digestion decreases at low rumen pH, especially below pH 6.0, as observed previously in studies using continuous cultures of mixed ruminal micro organisms, in vitro rumen, and in situ techniques. The objective of the present experiment was to investigate the effect of fluctuation in rumen pH on the attachment of Ruminococcus flavefaciens to feed particles incubated in the rumen.