A culture from a single cell of a flocculent brewing yeast became less flocculent during subculture on agar slopes. It was found that the culture had become a mixture of a number of variants differing in degree of flocculence. Five of the variants were studied using a sedimentation method to measure flocculence. During subculture on agar and in liquid medium, single cell cultures of all these variants except the least flocculent gave rise to other variants. The proportion of less flocculent yeast increased during further subculture. The change was faster in liquid medium than on agar slopes. Yeast with more stable flocculence could be selected from the part of the population which did not change. Experiments with mixtures of 'stable' yeast indicated that the less flocculent yeast had a selective advantage, and that the mechanism of the selection involves the different physical properties of the yeast. All variants examined had the same rate of growth. Cultures derived from single cells of a given variant tended to give rise to the same new variants. Yeast with flocculence similar to the original strains could be isolated from the variant strains. It was not possible to determine whether these variants were produced by gene mutation since spore formation has never been detected in this yeast.