Ruminal ciliated protozoa are similar to bacteria in that their numbers increase in response to increased substrate availability. Moderate increases in grain intake result in large increases in the numbers of ciliated protozoa (Dennis et al., 1983). In contrast, high-or all grain diets have variable effects, including total elimination of protozoa in some animals (Franzolin and Dehority, 1996). Ciliated protozoa are believed to be much more sensitive than bacteria to fluctuations and reductions in ruminal pH; hence, ruminal pH is a critical factor in the maintenance of ciliated protozoa in the rumen. The objective of the present experiment was to investigate the effect of fluctuation in ruminal pH on the population of protozoa in rumen fluid.