On the basis of the levels of secondary product formation four different phenotypes were represented among the 28 strains of Saccharomyces cerevisiae isolated during the spontaneous fermentation of grape juice. The genetic analysis indicated that four different strains, representing each phenotypic class, were derived, one from the other, by mutation. The spontaneous fermentation of a Malvasia must was dominated by different strains of Saccharomyces cerevisiae at different stages of fermentation.