A yeast (Saccharomyces cerevisiae) deletion library was screened based on NADH fluorescence using a 384 well plate assay and robotics to identify a yeast isolate lacking the 24 min periodic cell surface oxidase. The oxidase was shown previously to be a candidate ultradian oscillator of the yeast’s biological clock. The cDNA was cloned from a yeast overexpression library and the encoded protein was expressed in bacteria and characterized. Glyceraldehyde-3-phosphate dehydrogenase activity was used as the cellular circadian indicator. The identified gene was YML117W which encodes a ca 126 kDa putative RNA-binding protein. The candidate ENOX1 activity from yeast had functional characteristics similar to those of other constitutive ENOX1 proteins of eukaryotes exhibiting oscillating activities with a temperature independent period length of 24 min phased by melatonin and low frequency electromagnetic fields and susceptible to inhibition by the ENOX1 inhibitor, simalikalactone D. The YML117W deletion mutant cells lacked the ENOX1 clock output present in wild type yeast. The findings identify YML117W as the ENOX1 of Saccharomyces cerevisiae and support its proposed function as an ultradian oscillator of the yeast biological clock.