Chronic selenium toxicosis was induced in 1-yr-old male mallard ducks (Anas platyrhynchos) by feeding selenium, as seleno-DL-methionine, in amounts of 0, 10, 20, 40, and 80 parts per million (ppm) to five groups of 21 ducks each for 16 wk during March to July 1988. All mallards in the 80 ppm group, three in the 40 ppm group, and one in the 20 ppm group died. Histologic lesions in mallards that died of selenosis were hepatocellular vacuolar degeneration progressing to centrolobular and panlobular necrosis, nephrosis, apoptosis of pancreatic exocrine cells, hypermaturity and avascularity of contour feathers of the head with atrophy of feather follicles, lymphocytic necrosis and atrophy of lymphoid organs (spleen, gut-associated lymphoid tissue, and lumbar lymph nodes), and severe atrophy and degeneration of fat. Histologic lesions in surviving mallards in the 40 ppm group, which had tissue residues of selenium comparable to mallards that died, were fewer and much milder than mallards that died; lesions consisted of atrophy of lymphoid tissue, hyalinogranular swelling of hepatoeytes, atrophy of seminiferous tubules, and senescence of feathers. No significant histologic lesions were detected in euthanized mallards in the 0, 10 and 20 ppm groups. Based on tissue residues and histologic findings, primarily in the liver, there was a threshold of selenium accumulation above which pathophysiologic changes were rapid and fatal. Pathognomonic histologic lesions of fatal and nonfatal selenosis were not detected. Criteria for diagnosis of fatal selenosis in aquatic birds include consistent histologic lesions in the liver, kidneys, and organs of the immune system. Although histologic changes were present in cases of chronic non-fatal selenosis, these were inconsistent. Consistent features of fatal and non-fatal chronic selenosis were marked weight loss and elevated concentrations of selenium in organs.