Samharomycea cerevisiae and Saccharomycea carlsbergensis were grown in batch culture with and without oxygen control.The concentrations of A-, E and C-type cytochromes of both yeasts were dependent on the oxygen concentration during growth as well as on the initial glucose concentration of the growth medium. S. cerevisiae cytochromes were maximal after growth in low glucose and low oxygen; S. curlsbergensis cytochromes were maximal after growth in low glucose and high oxygen. Except when glucose was in very low concentration, its catabolism by 8. carlsbergensis was directed predominantly towards ethanolic fermentation regardless of the oxygen concentration. Growth rate, tot,al cell msss and yield were maximal, and anabolism was closely balanced with catabolism, when glucose and oxygen of S. carlsbergensis cultures were both high. Under these conditions neither catabolism, respiratory or ethanolic, nor glucose uptake were maximal.