In previous work the rumen microbiology of young animals fed on a high carbohydrate ration has been considered and has tended to be fairly consistent (Eadie, Hobson and Mann, 1959; Eadie, 1962). In this note a further controlled experiment on calves weaned on to a 50:50 mixture of roughage/high concentrate mixture and on to the high concentrate alone, is reported. In view of interest in the intensive feeding of cattle on barley these results from young animals are compared with a series of observations made more recently on older animals fed predominantly on barley, since it was obvious from the outset that the physical nature of the rumen contents was different in the two age groups. The sudden introduction of large amounts of carbohydrate into the diet of animals previously fed conventionally has been shown to create profound changes in the rumen microbial population (Krogh, 1961) and the microbiology of the young concentrate-fed calf was very different from the roughage-fed animal. However, the barley-fed steers used in this work had been early weaned on to a high concentrate ration and had clearly become adapted to the adult high carbohydrate diet.