British brewery yeasts normally consist of mixtures of flocculent (Burns value > 1.4) and non-flocculent (< 0.1) strains, the degree of fluocculence and hence the attenuative power of a particular sample depending on the relative proportions of the two. Pure-strain yeasts of intermediate fluocculence have not been found. Advantage can be taken of the characters of the Burton Union system of fermentation to maintain stocks of yeast of desired flocculence and attenuative character, the method depending on selection of non-flocculating cells, with mutation and differential growth rates playing a minor part in determining the proportions of flocculating and non-flocculating cells. With continuous propagation, where substantial fractionation cannot occur, mutation and differential growth rates are the predominant factors leading to change.