The net benefit that Saccharomyces cerevisiae obtains from aerobiosis as compared to anaerobiosis has been studied. For this purpose yeasts with different respiratory capacities have been obtained by growing them in batch cultures on different substrates. Even with sugars with low catabolite repression effect, as is the case of galactose, aerobiosis increased the growth rate and the growth yield by less than two-fold. These variations, which are much lower than the expected considering the actual oxygen utilization, indicate that either the amount of ATP produced in respiration is much lower than the theoretically expected or a much greater expenditure of ATP occurs in aerobic than in anaerobic growth. The results show that S. cerevisiae obtains only a slight benefit from aerobiosis when growing on sugars at the relatively high concentration prevailing in its natural habitats.