1. Samples of mixed bacteria were separated from rumen digesta taken from calves, kept out of contact with adult animals, and from sheep and cows.
2. For calves receiving a diet made up of equal amounts of roughage and cereals with 13–16 g nitrogen/kg dry matter, samples of mixed bacteria taken 4–6 h after feeding contained, on average, 140 g glucose in α-linked polymers (α-dextran), 25 g galactose and a total of 25 g other non-glucose, non-galactose sugars (mainly rhamnose, ribose and mannose) in combined forms per kg dry matter.
3. The α-dextran content of similar bacteria samples from sheep or cows receiving diets of similar composition was 70 g/kg dry matter. Samples from animals receiving all-roughage diets contained only 25 g α-dextran/kg dry matter, but those from cows given more than 70% of their ration as concentrates (mainly cereal) contained 150 g α-dextran/kg dry matter.
4. Addition of supplementary protein or urea to cereal–roughage diets given to calves greatly depressed the amount of α-dextran in the rumen bacterial samples to an average value of 60 g/kg dry matter.
5. Samples taken before a morning feed (i.e. after 16 h fasting) contained less α-dextran than samples taken 4–6 h after feeding for both calves and cows.
6. Under different conditions, variations in the amounts of galactose in rumen bacteria sometimes paralleled variations in α-dextran. Amounts of other non-glucose sugars did not vary greatly.
7. It was estimated, from a comparison of the compositions of rumen bacteria and duodenal contents, that, in the latter, the rhamnose, ribose and mannose came mainly from the bacteria, the arabinose, xylose and cellulose-glucose mainly from the diet and the galactose and α-dextran-glucose from both sources.