The products of C(14)O(2) fixation by Chlamydomonas and Chlorella were studied under conditions most favorable for glycolate synthesis. The highest percentage of the C(14) was incorporated into glycolate in the pH range of 8 to 9. After 1 to 2 minutes as much as 40% of the C(14) was found in glycolate products and only a trace of C(14) was present as phosphoglycerate. Below pH 8 the rate of photosynthesis was much faster, but only a small percent of the C(14) was incorporated into glycolate in 1 or 2 minutes, while a high percent of the C(14) accumulated in phosphoglycerate. C(14) labeling of glycolate even at pH 8 or above did not occur at times shorter than 10 seconds. During the first seconds of photosynthesis, nearly all of the C(14) was found in phosphoglycerate and sugar phosphates. Thus glycolate appears to be formed after the phosphate esters of the photosynthetic carbon cycle.Washing Chlamydomonas with water 2 or 3 times resulted in the loss of most of their free phosphate. When a small aliquot of NaHC(14)O(3) was added to washed algae in the absence of this buffering capacity, the pH of the algal medium became 8 or above and much of the fixed C(14) accumulated in glycolate.