A preadaptive purely aerobic utilization of galactose by yeast cells has been demonstrated. Hence, the adaptation by yeast to galactose is not to its utilization per se, but specifically to its metabolism by a glycolytic mechanism. An examination of this preadaptive oxidation of galactose reveals that it has many characteristics in common with the endogenous metabolism of yeast. Included among these are the similarities of the R.Q. values and the response of the Q(O(O2) ) and Q(CO(CO2) ) (O(2) ) to KCN and iodoacetic acid. Further, a competitive interaction appears to exist between the endogenous respiration and the preadaptive oxidation of the galactose. The latter can replace the endogenous respiration as a source of energy for the adaptation to the fermentation of the galactose. Carbon balance studies of the galactose oxidation revealed that polysaccharide could be formed as a result of this metabolism during the preadaptive period. Non-adaptable cells were also found to possess the capacity to oxidize galactose in the complete absence of any ability to metabolize it anaerobically. The significance of these findings for the biochemistry and physiology of the adaptation is discussed.