The toxicities of seleno-l-methionine, seleno-dl-methionine, and selenized yeast were compared. Ten pairs of mallards were fed a control diet and 15 pairs were fed diets containing 10 ppm selenium as seleno-dl-methionine, seleno-l-methionine, or selenized yeast. Hatching of fertile eggs was significantly lower for females fed 10 ppm selenium as seleno-dl-methionine (7.6%) and seleno-l-methionine (6.4%) than for controls (41.3%). Survival of ducklings was lower when their parents had been fed 10 ppm selenium as seleno-l-methionine (20.0%) than for controls (98.4%). The number of 6-day-old ducklings produced per female was significantly lower for mallards fed 10 ppm selenium as seleno-dl-methionine (0.47) or selenized yeast (2.67) than for controls (6.10), and was significantly lower for mallards fed seleno-l-methionine (0.13) than for mallards fed selenized yeast. The eighth eggs of females fed the dl or l forms of selenomethionine contained means of 9.2 and 8.9 ppm selenium, wet weight; these means were higher than the mean (6.6 ppm) for females fed selenized yeast. Among embryos that died at 7 days of age or older, the percentage of embryos that were deformed was 1.3% for controls, 24.6% for seleno-dl-methionine, 28.2% for seleno-l-methionine, and 11.0% for selenized yeast. The results suggested that seleno-dl-methionine and seleno-l-methionine were of similar toxicity and were both more toxic than selenium from selenized yeast.