Selenium, recognized as an essential nutrient for human health, is a component of proteins and enzymes required for various biological functions and is currently being used as a feed supplement for livestock in geographical areas that are naturally low in selenium. Selenium is structurally similar to sulfur, replacing the sulfur atom in stoichiometric amounts and thus functions through an association with proteins, termed selenoproteins. In geographic areas low in selenium, there is the potential for animals (including humans) to become selenium deficient and this potential deficiency can be remedied by consumption of exogenous selenium, including selenium-enriched yeast (Saccharomyces cerevisiae) that contains high levels of organic selenium (e.g., selenized yeast). A unique, standardized, registered high selenium food-grade baker's yeast (S. cerevisiae; Sel-Plex), was tested in the following battery of Genotoxicity assays; (1) a bacterial reverse mutation test (Ames test); (2) an in vitro mammalian chromosome aberration test; and (3) a mouse micronucleus test. Under the conditions of this assay, Sel-Plex showed no evidence of mutagenic activity in Salmonella typhimurium, in the bacterial reverse mutation test. Sel-Plex did not induce significant chromosomal aberrations in cultured human lymphocytes in the in vitro mammalian chromosome aberration test. Sel-Plex did not statistically increase the frequency or proportion of micronucleated immature erythrocytes in the mouse micronucleus test. Thus, from the studies presented here, the authors conclude that Sel-Plex is nongenotoxic.