Two phenotypically distinct sets of nystatin-resistant mutants were investigated. One set is resistant, respiratory competent, and requires no lipid for growth. The other set is more resistant, respiratory deficient, and lipid requiring (unsaturated fatty acid or sterol). Both sets show altered sterol composition as demonstrated by the Liebermann-Burchard colorimetric reaction, ultraviolet spectrophotometry, and gas-liquid chromatography. Genetic analysis indicates that all nystatin-resistant mutants can be placed into one of six distinct genetic groups. The phenotype's nystatin resistance, lipid requirement, and respiratory deficiency are recessive. There was one case of allelism for mutants from different sets. Revertants of mutants which have the tripartite phenotype retain a residual level of nystatin resistance, but they are no longer lipid requiring or respiratory deficient. Growth studies in mutants which have the tripartite phenotype reveal that the addition of ergosterol to the growth medium results in decreased resistance to nystatin.