This paper consists in part of a brief review of what has been learned about the bacteriology of chronic ulcerative colitis in the last ten years and will make the contribution of correlating the living and the postmortem pathology. The first part deals with bacteriology, the second with the proctoscopic picture, and the third with the microscopic changes in tissue. BACTERIOLOGY We have learned to recognize chronic ulcerative colitis as a clinical entity and to recognize a microorganism which we believe plays a part in causation of most cases. The results of the activities of this organism have been seen in the bowel of man, and the organism can be recognized by its appearance, its cultural characteristics and its biologic reactions. It has been dealt with in the laboratory for almost a decade. We believe its connection with chronic ulcerative colitis has been established by fulfilment of those requirements accepted