The apparent photosynthetic Km (CO2) of air-grown Dunaliella salina is 2 μM as measured both by the filtering centrifugation technique and by O2 electrode. These cells are capable of accumulating inorganic carbon (Cinorg) up to 20 times its concentration in the medium. It is suggested that air-grown Dunaliella cells are able to concentrate CO2 within the cell. Analysis of the efflux of Cinorg from cells previously loaded with H(14)CO 3 (-) demonstrated the existence of an internal pool which has an half-time of depletion of 2.5-7 min depending on the conditions of the experiment. This finding indicates that the internal Cinorg pool is not readily exchangeable with the external medium. Furthermore, the influence of the presence or absence of unlabelled Cinorg in the medium during the efflux experiment on the half-time observed indicate that efflux of Cinorg is not a simple diffusion process but is rather carrier-mediated.