The toxicity of four chemical forms of selenium (seleno-L-methionine, seleno-DL-methionine, selenized yeast, and high selenium wheat) was compared in day-old mallard ducklings (Anas platyrhynchos). In the first experiment, in which the basal diet was 75% wheat, survival after 2 weeks was lower for ducklings fed 30 μg/g selenium as seleno-L-methionine (36%) than for ducklings fed 30 μg/g selenium as seleno-DL-methionine (100%) or 30 μg/g selenium from high selenium yeast (88%). The concentration of selenium at 2 weeks in the livers of survivors was similar for ducklings fed 15 μg/g selenium as seleno-DL-methionine (12 μg/g, wet weight), seleno-L-methionine (11 μg/g), and high selenium wheat (11 μg/g), but was lower when the selenium came from selenized yeast (6.2 μg/g). When fed 30 μg/g selenium from the various sources, the selenium concentrations in liver were 20 μg/g for seleno-DL-methionine, 19 μg/g for seleno-L-methionine, and 9.9 μg/g for selenized yeast. In a second experiment, in which the basal diet was a commercial duck feed, survival after 2 weeks was 100% in ducklings fed 30 μg/g selenium as seleno-DL-methionine, seleno-L-methionine, or selenized yeast. Selenium concentrations in liver were similar for ducklings fed the 30-μg/g selenium diets as the DL or L forms of selenomethionine (27 and 25 μg/g), but lower for ducklings fed selenized yeast (13 μg/g). The greater toxicity of the L form of selenomethionine was probably related to the palatability or nutritional nature of the wheat-based diet used in experiment 1, but the exact reason for the difference between the DL and L forms is unknown. Biologically incorporated selenium, derived from high selenium wheat, was no more toxic than selenium derived from the two purified forms of selenomethionine, and the selenium in selenized yeast was not as toxic as that in the two forms of selenomethionine.