Abstract: Eukaryotic algae exhibit marked differences in photosynthetic carbon metabolism dependent on the CO2concentrationduring growth. The lower affinity for inorganic carbon, high CO2compensation and decreased carbonic anhydrase in high CO2con-centration have been reported. However, little attention has been paid to the response of PSII to elevated CO2concentration in a-lgae. Chlorophyll a fluorescence has been widely used to study thePSII behavior under different stress conditions which results inthe establishment of different physiological states. A suitable cell model to study the CO2effects in algae is the protist Chlamy-domonas reinhardtii. To investigate the influences of external high CO2concentration on the energy fluxes and yield of PSII, thechanges of PSII behavior in thegreen alga Chlamydomonas reinhardtii cells exposed to high CO2concentration for 12h were stud-ied by analyzing the chlorophyll a fluorescence transients. High CO2concentration resulted in significant increase by 16.1%,13.7%and 13.9%in the content of chlorophyll a, b and carotenoids, respectively. The polyphasic fluorescencetransients(O-J--IP)was measured by using Plant Efficiency Analyser (PEA) after the cells were dark-adapted, and showed that the level at O, J, Iand P in high-CO2-grown cells significantly raised with the increasing exposure time compared with air-grown ones. The relativevariable fluorescence at the phase J (VJ) maintained a higher value in high-CO2-grown cells in contrast to air-grown ones. Thespecific energy flux for absorption(ABS/RC) after cells were cultured in high CO2concentration (4%CO2) for 6 h had a markedincrease,whereas the specific energy fluxes for electron transport (ETO/RC) and trapping (TRo/RC)were not affected by high CO2concentration. The maximal quantum yield of primary photochemistry (UPo),the efficiencywith which a trapped exciton can movean electron into the electron transport chain further than QA (W O), and the quantum yield of electron transport (UEo) were lower inhigh-CO2-grown cells than in air-grown ones. These results in this study suggested that the difference of chlorophyll a fluores-cence reported here reflected the changes of PSII function which may be associated with active transport of HCO3. The analysisof polyphasic chlorophyll a fluorescence transients was a powerful tool to study the changes of the energy fluxes and yield of PSIIin high CO2concentration.