A relation between ethanol production and flocculation at high temperature was studied on J a ccharomyces cerevisiae bred for ethanol production from beet molasses. As thermal death of dispersed yeast cells was larger than that of flocculent yeast cells at 40°C and 50°C, it was suggested that the ability to flocculate was one of factors to promote ethanol production at high temperature. Fermentation test of beet molasses indicated that flocculent strain M-9 produced ethanol at the concentration of 7. 2% and 3. 2%, at 40°C and 50°C, respectively. he flocculating property of segregants of strain M-9 was destroyed by chymotrypsin digestion and was stable to incubation at 70°C. These data and tetrad analysis of a cross between a segregant of strain M-9 and FLO 1 strain or FLO 5 strain suggested that flocculation of strain M-9 was dominated by FLO 1 gene.