The effect of sodium selenite on cell number increase was studied in the yeasts Saccharomyces cerevisiae and Candida tropicalis. It was found that C. tropicalis was more susceptible to the toxicity of selenium than S. cerevisiae. The presence of selenium in the medium did not reduce the yeast' demand for sulfate ions, and the magnitude of the toxic selenium dose depended on the sulfur in the medium and on the kind of yeast.