Isogenic strains of yeast were constructed, differing only in insertionally inactivated genes for ergosterol biosynthesis. These and their allelic wild-types were grown in competition to ascertain growth differences and any selective advantage for organisms producing sterols with or without specific features of ergosterol. In every instance tested, the wild-type allele afforded a competitive advantage over the isogenic pair producing modified sterol structures instead of ergosterol. A general trend was seen in which the earlier in the biosynthetic pathway that a mutation occurred, the less able the strain producing the defective sterols could compete with the ergosterol-producing strains.