Activity of carbonic anhydrase (CA) was studied in wild type (WT) Chlamydomonas reinhardtii and five mutants distinguished only by retaining photosystem 1 (PS 1), photosystem 2 (PS 2), light-harvesting complexes (LHC) of PS 1 or PS 2 and both photosystems without LHC. Two forms of CA, soluble CA (sCA) and membrane- bound (mbCA), were found in the cells. The highest activity of mbCA was obseiwed in mutants which retained both photosystems, oř only PS 1 and PS 2. The mutants deprived of photosystems but retaining LHC almost lacked mbCA. Thus, there was a correlation between the retention of mbCA and mutant abilities to perform the photosynthesis. Photosynthetic characteristics of the high CO2 requiring mutant of C. reinhardtii CIA-3 were compared with those of WT grown at 2 % CO2 and after various times of exposure to limiting CO2 concentration (0.03 %). CIA-3 cells háve a lower photosynthetic affmity for inorganic carbon (Cj) as compared to WT when grown at high and low CO2 concentrations. Only in the WT cells the photosynthetic affmity for Cj was increased when the gas phase was changed to air. In CIA-3 the photosynthesis was inhibited by a high concentration of Cj.