The rumen population of the ox comprises three major groups of micro-organisms:(a) Protozoa, (b) iodophile micro-organisms, (c) aniodophile micro-organisms. The iodophile species comprise macro and micro forms. The forms include several new species and genera which are described in the paper. A fixed and a free iodophile population are discriminated, the former constantly attached to starch grains and vegetable fragments, the latter in suspension in the rumen liquid. The rumen population was qualitatively independent of the range of diet and of the breed of animal examined. Protozoa play no part in the decomposition of cellulose but digest starch. Decomposition of both starch and cellulose is accomplished by the fixed iodophile microflora which, in the process, synthesize iodophile polysaccharide. During passage through the alimentary canal the rumen population is eliminated, the Protozoa by peptic and tryptic digestion and the iodophile microflora by (a) digestion by Protozoa, (b) autolysis, (c) action of digestive enzymes. The products of microbial synthesis are held to be assimilated by the host animal.