A yeast flocculation gene was isolated from a genomic library of an FLO5 strain of S. cerevisiae on the basis of its ability to trigger flocculation in a non-flocculent strain. Characterization of the cloned gene by restriction mapping, Southern analysis, and chromosome mapping have shown that it corresponds to a FLO5 gene previously located on chromosome I and that this gene is related to the already described FLO1 gene. A study of gene expression in different yeast strains has indicated that, while this gene is dominant, its expression can be suppressed in some genetic backgrounds. A Northern-blot analysis has demonstrated that the same 5000-nt transcript was present in an FLO5 and an FLO1 strain. A gene disruption experiment has led to the conclusion that another flocculation gene is present and can be active in the FLO5 strain we used.