Abstract: Some physiological charactenshcs of a mutant(No.150) of SynechococcusPCC7942, which reqruires very high CO2(10%) for growth, were studied.The mutantwas isolated after ethylmethyl sulphonate(EMS) mutagenesls and ampicillin enrichment Unlike other cyanobacterial high CO2 requiring mutant,where the apparent photosynthetice affinity for inorganic carbon is approximately 2 oders of magnitude lower than that of the wild type, the mutant exhibits photosynthetic chaxactensticssimilar to those of its wild type grown at high CO2(Fig 2).Furthermore,the Ciinflux and efflux rate,and Ci pool of the mutant are two and three times tigher thanthat of the wild type grown at high CO2, respectively(Table 1).It was later foundthat with added NaCl at a concentration no less than 100mmol／ L, the mutant couldgrow equally Well in air containing 1% CO2 as that in air containing 10% CO2.It issuggested that Sodium could partly alleviate the demanding of the mutant for veryhigh CO2 The results indicated thgt mutant Is msnly defeCted in the ability to adapt to low CO2 conditions.