With Chlorella ellipsoidea cells, the effect of oxygen was investigated on the products of enhanced dark (14)CO(2) fixation immediately following preillumination in the absence of CO(2). When the reaction mixture was made aerobic by bubbling air (CO(2)-free) throughout preillumination and the following dark (14)CO(2) fixation periods, the initial fixation product was mainly 3-phosphoglyceric acid. When nitrogen gas had been used instead of air, only about one-half of the total radioactivity in the initial fixation products was in 3-phosphoglyceric acid and the rest in aspartic, phosphoenolpyruvic, and malic acids. The percentage distribution of radioactivity incorporated in these initial products rapidly decreased during the rest of the dark period. Concurrent with the decrease in the initial (14)CO(2) fixation products, some increase was observed in the radioactivities of the sugar phosphates. The maximal radioactivity incorporated in sugar mono- and diphosphates accounted for only 10% of total (14)C, under either the aerobic or anaerobic conditions. Under anaerobic conditions most of the (14)C incorporated was eventually transferred to alanine, whereas the main end products under aerobic conditions were aspartate and glutamate. The pattern of (14)CO(2) fixation products was unaffected by the atmospheric condition during the period of preillumination. The preferential flow of the fixed carbon atom to alanine or aspartate depended on the presence or absence of oxygen during the period of dark CO(2) fixation.