Adaptation of yeast torulopsis candida to D-arabinose, D-lyxose and L-oxylose is shown to be due to the appearance of a new enzyme which catalyzes the reduction of these pentoses to D-arabitol and xylitol, respectively. The appearance of the enzyme is not coupled with the altered state of polyol : NADP-oxidoreductase (1.1.1.21) catalyzing the reduction of D-xylose and other aldoses with their second carbon atom having a stereoconformation of D-glyceraldehyde. Some proposals are made on improvements of the nomenclature of the enzymes catalyzing the reduction of the two groups of pentoses.