Abstract The chlorophyll a fluorescence yield of the cyanobacterium Synechococcus UTEX 625 decreased upon the initiation of inorganic carbon transport. The fluorescence yield recovered upon the depletion of inorganic carbon from the medium or upon the addition of DCMU. The inhibition of photosynthetic CO 2 fixation by iodoacetamide did not prevent this reduction of fluorescence yield. Similar results were obtained for both Na +-stimulated HCO 3 − transport and for the transport (presumably of CO 2) that is stimulated by carbonic anhydrase. A transient lowering of the fluorescence yield was also observed when cell suspensions were pulsed with CO 2. In cells not inhibited with iodoacetamide, a very close quantitative relationship existed between the net rate of O 2 evolution and the maximum extent of fluorescence quenching seen as a function of the inorganic carbon concentration. The fluorescence quenching, however, was not due to CO 2 fixation but rather to the transport of inorganic carbon or the accumulation of the internal pool of inorganic carbon. If quenching is due to the latter it is not surprising that the extent of quenching corresponds to the maximum rate of photosynthesis as the rate of photosynthesis also depends on the size of the internal pool. The results with DCMU suggest that the quenching is Q quenching and transport must provide a mechanism for the oxidation of Q other than CO 2 fixation.