In our previous study, we elucidated for the first time that yeast mitochondria fragment during alcohol fermentation (5) but not the mechanism of mitochondrial fragmentation during sake brewing. Therefore, here, we first attempted to identify the factor involved in mitochondrial fragmentation during alcohol fermentation. A mitochondrial fission protein, Fis1p, has been reported to be responsible for mitochondrial fragmentation during vegetative growth (20) and acute ethanol stress (4) in yeast. Therefore, we investigated whether disruption of the FIS1 gene leads to inhibition of mitochondrial fragmentation during alcohol fermentation. Sake was brewed for 22 d with either WT or fis1Δ of a laboratory strain (BY4743) harboring mitochondria-targeted GFP ( 21 and 22), and mitochondrial morphology was visualized by fluorescence microscopy. Sake brewing proceeded without any observable defects, as verified by the final decrease in the weight of the mash (WT: 32.23 g, fis1Δ: 33.83 g). Although WT mitochondria fragmented as the brewing proceeded ( Fig. 1A), as we reported previously ( 5), fis1Δ mitochondria remained networked until the end of brewing ( Fig. 1A). This result clearly indicates that Fis1p is responsible for mitochondrial fragmentation during sake brewing.