Abstract: The brewer's yeast genome encodes a 'Flo' flocculin family responsible for flocculation. Controlled floc formation or flocculation at the end of fermentation is of great importance in the brewing industry since it is a cost-effective and environmental-friendly technique to separate yeast cells from the final beer. FLO genes have the notable capacity to evolve and diverge many times faster than other genes. In actual practice, this genetic variability may directly alter the flocculin structure, which in turn may affect the flocculation onset and/or strength in an uncontrolled manner. Here, 16 ale and lager yeast strains from different breweries, one laboratory Saccharomyces cerevisiae and one reference Saccharomyces pastorianus strain, with divergent flocculation strengths, were selected and screened for characteristic FLO gene sequences. Most of the strains could be distinguished by a typical pattern of these FLO gene markers. The FLO1 and FLO10 markers were only present in five out of the 18 yeast strains, while the FLO9 marker was ubiquitous in all the tested strains. Surprisingly, three strongly flocculating ale yeast strains in this screening also share a typical 'lager' yeast FLO gene marker. Further analysis revealed that a complete Lg-FLO1 allele was present in these ale yeasts. Taken together, this explicit genetic variation between flocculation genes hampers attempts to understand and control the flocculation behavior in industrial brewer's yeasts.