Characteristics of the killer yeast No.337, which was isolated from an alcohol fermenting mash in Brazil, were described. This yeast was classified into Saccharomyces cerevisiae from its fermentability and nitrogen assimilbility and so on, but its killer properties were different from those of killer Saccharomyces strains hither to reported. This killer gene was carried on a double-stranded RNA plasmid. Especially, this strain exhibited a strong killer activity against an alcohol fermention strain, Itaquara No.2. From effects of pH on its killer activity and its resistance against K 1 killer toxin, it was suggested that this killer was similar to the killer K1 type.