1. In a study of the microbial population in the rumen contents of sheep fed on a semipurified or a roughage-concentrate diet, the total counts and morphological groups of bacteria and protozoa, the counts of proteolytic, amylolytic and cellulolytic bacteria and the rates of breakdown of cellulose and starch in vitro were determined. Three sheep received each diet. 2. Protozoa disappeared completely from the rumen of sheep fed on the semi-purified diet. High counts of Entodinia persisted in the rumen of two sheep on the roughage-concentrate diet; a third animal was maintained defaunated on this diet. 3. The mean total counts of bacteria per g of rumen contents were 5 x 1010 and 2.5 x 1010 respectively in sheep fed on the semi-purified and roughage-concentrate diets and 11.4 x 1010 in the defaunated sheep. 4. The proportions of the morphological groups of bacteria and the counts of amylolytic bacteria were similar with both diets; the mean counts of proteolytic and cellulolytic bacteria were twice as high in the sheep on the semi-purified diet. The counts of all three functional groups of bacteria were considerably higher in the single defaunated sheep. 5. The mean rates of cellulose breakdown were 16.6 and 9.5 g/l. rumen fluid per 24 h for the sheep fed on the semi-purified and roughage-concentrate diets respectively. The corresponding rates of starch fermentation were 28 and 42.4 g/l. rumen fluid per 24 h. 6. It is concluded that the bacterial population in the rumen of sheep fed on the semi-purified and the roughage-concentrate diets differed quantitatively rather than qualitatively. It is also concluded that the absence of protozoa, rather than a direct nutritive effect of the semi-purified diet, was responsible for the increased bacterial population in the rumen of the sheep fed on the semi-purified diet. The much higher bacterial counts in the rumen of the defaunated sheep support this view.