Air-grown cells of the cyanobacterium, Coccochloris peniocystis Kutz were exposed to [(14)C] bicarbonate in the light for periods of 0.5 to 2.0 s followed by longer exposures to unlabelled bicarbonate. Although C4 acids are among the initial products of photosynthesis, the kinetics of tracer movement during the pulse-chase experiments demonstrate that the principal mechanism of CO2 fixation in this alga is the C3-pathway.