The cecal contents of normal rabbits and rabbits with mucoid enteropathy (ME) were examined microscopically. Rabbits with ME consistently had dramatic cecal dysbiosis characterized by a loss of protozoa, large metachromatic bacilli and other gram-positive organisms and an increase in the size and stain retention of gram-negative organisms. Experimentally induced cecal hyperacidity in fistulated rabbits produced dysbiosis similar to that observed in natural cases of ME. Cecal pH measurements in normal young and adult rabbits revealed that pH values consistently were lower in young rabbits and frequently were low enough to induce microbial changes. Spontaneous cecal hyperacidity in young rabbits, therefore, appears to account for the cecal distention and diarrhea seen initially in ME. Late manifestations such as mucus hypersecretion and impaction, on the other hand, appear to be the result of specific microbial factors which develop when dysbiosis persists. The gradual change in the dysbiotic flora to one which closely resembles that of the normal rabbit colon appears to be the stimulus that permits these late manifestations to develop.