Contents of the terminal ilea, ceca-proximal colons, and terminal recta were obtained from nine sheep, three of which were fed 100% orchardgrass hay, three of which were fed 60% cracked corn-40% orchardgrass hay, and three of which were fed 80% cracked corn-20% orchardgrass hay. The digestibility of dry matter in the ceca was greatest when the all-hay diet was fed, whereas the percentage of cellulose digestion in the ceca increased with increasing levels of concentrate. For all diets, the total volatile fatty acid concentrations were higher in the ceca than in the other two sites. The cecal pH levels decreased with increased corn levels in the diet. The total microbiol numbers per gram of ileal and cecal contents increased in response to feeding of concentrate; however, across all diets, the ileal counts were 8% or less of the cecal counts. In contrast, the cellulolytic microbial numbers in the ilea were 50% or more of those in the ceca and were highest with the all-hay diet. Both bacterial and end product concentrations in the ceca were equivalent to those occurring in rumina. A total of 16 cellulolytic cultures were isolated and characterized from ileal and cecal contents of animals fed all three diets. Seven gram-negative-rod-shaped organisms were identified as Butyrivibrio fibrisolvens, whose capacity to digest cellulose exceeded that of several rumen strains. The guanine-plus-cytosine content of the DNA of one strain of B. fibrisolvens was 38.8 mol%, compared with the only previously reported value for this species of 41.2 mol%.(ABSTRACT TRUNCATED AT 250 WORDS)