SUMMARY: A simple medium and dilution count technique was evolved for determining the viable count of rumen bacteria. Material from three fistulated cows was examined and the viable population found to vary within the range 107-1012/g; the most frequently occurring values were 108-1010/g (94.0% of 350 counts). Observations were made of the homogeneity of rumen contents, of variations in count with time after sampling and between cows, and of variation from day to day, hour to hour, before and after feeding and on different diets. The variations in the viable count, even with gross changes in diet, were never greater than those occurring in a single animal on a fixed diet. Microscopic examinations showed Gram-positive cocci and rods to be the most frequently occurring types of bacteria, Gram-negative organisms of the coli-aerogenes type being present only in low numbers.