1. The composition of the bacterial populations of the rumen of three sheep on lucerne hay was very similar with regard to both the levels of the functional groups which ferment cellulose, starch, glucose and xylose and to the morphological types of which they were composed. The lactate-utilizing bacteria showed more variation in numbers between individual animals.
2. The rumen flora of lucerne hay fed sheep differed from that of sheep on teff hay not only in significantly higher counts of the carbohydratefermenting bacteria in the former group, but also in distinctive types of cellulolytic bacteria.
3. The short- and long-term variations in number of cellulose digesters found earlier in a sheep on a practical diet of lucerne hay were not markedly reduced by feeding the same animal lucerne hay of constant composition.
4. A correlation appeared to exist between the protein content of the lucerne hay and the level of the lactate-fermenting bacteria in the rumen.
5. The changes in the viable counts of the bacteria capable of fermenting cellulose, starch, glucose and lactate during the first 8 hr. after feeding a ration of lucerne hay were small compared with the in vitro rate of multiplication of a pure strain of cellulolytic cocci isolated from a sheep on the same diet. This is regarded as further evidence in favour of the notion that the rumen contents, particularly of animals on high roughage diets, approximates to a continuous culture system.