Strains of Saccharomyces cerevisiae (NCYC 1190, 1214 and 1364) behaved as nonflocculent in defined culture medium, as well as the strain NCYC 1214 in rich medium. Flocculation was induced by Ca2+ addition and/or by correcting the pH to a suitable value. Since the free Ca2+ concentration is pH-dependent, these two factors (total Ca2+ concentration and pH) cannot be dissociated and are the critical parameters governing flocculation, in culture medium, of the brewing strains studied.