Brook trout were fed various meat and meal mixtures to which vitamins A, B1, C, D, G, and PP were added. None of these diets or vitamins affected the resistance of the trout to ulcer disease. Trout on a “natural” diet of insects, minnows, etc., succumbed to the disease. Infected fish planted in a small stream plentifully supplied with food organisms continued to die from ulcer disease. Ulcers have been found on wild fish, and on hatchery fish a year after planting in natural waters. Feeding diseased tissue did not prove a more certain method of infecting trout than did contact with diseased individuals. Ingestion of diseased tissue, controlled to prevent external contamination, did not produce ulcer disease in brook trout yearlings. Trout were infected through contact of an open wound with kidney tissue from a diseased fish. An attempt to prevent ulcer disease by drastic treatment with potassium permanganate was unsuccessful. Data are presented on the effect of low temperatures in retarding the disease. Strains of brook trout from hatcheries long infected proved much more resistant to the disease than did other strains. Ulcer disease and fin rot have been shown to be identical diseases.