Cattle are often transitioned from a forage diet to a high-grain diet in order to increase available feed energy for milk production. This transition can result in depressed ruminal pH and subacute ruminai acidosis (SARA). Changes in the diet can also impact the rumen microbiota. This thesis investigates how a transition from a hay to a high-grain diet effects the diversity and density of bacteria, methanogens, and protozoa, as well as how diet change can affect the distribution of rumen protozoa. Four non-lactating dairy cows were followed for seven weeks through one week of hay feeding, a transition to 65% grain diet for three weeks, and a switch to hay feeding for an additional three weeks. Ruminai pH was depressed below 5.6 for 4.6h/day in week 1, and below 6.0 in weeks 2 and 3 for 4.8h/day and 3.4h/day, respectively. Weekly samples were collected from rumen fluid, rumen solids, and feces. Quantitative real-time PCR found no significant difference in the rumen fluid for bacterial or methanogen density at any time point, but protozoal density increased significantly in the rumen fluid during high-grain feeding. In the rumen solid, bacterial density significantly decreased during high-grain feeding, while it significantly increased for protozoa during the same time period. Based on 16S rRNA gene clone libraries, methanogen and bacterial community structure was significantly different for the high-grain period in the rumen fluid. All samples of rumen fluid examined had a type A protozoal population, which was maintained throughout the experiment. A linear programming model was also developed to predict rumen protozoal distribution between the free-living and attached phases. The proportion of protozoa in the attached phase ranged from 63-90%, depending on the diet, with more protozoa in the attached phase with hay feeding. This study shows that depressed ruminal pH due to high-grain feeding can significantly affect the density and community structure of the rumen microbiota, as well as the distribution of rumen protozoa.