The growth and activity of photosynthetic CO2 uptake and extracellular carbonic anhydrase (Caext) of the marine diatom Skeletonema costatum were investigated while cultured at different levels of CO2 in order to see its physiological response to different CO2 concentrations under either a low (30 μmol·m-2·s-1) or high (210 μmol·m-2·s-1) irradiance. The changes in CO2 concentrations (4-31 μmol/L) affected the growth and net photosynthesis to a greater extent under the low than under the high light regime. Caext was detected in the cells grown at 4 μmol/L CO2 but not at 31 and 12 μmol/L CO2, with its activity being about 2.5-fold higher at the high than at the low irradiance. Photo- synthetic CO2 affinity (1/ K1/2(CO2)) of the cells decreased with increased CO2 concentrations in culture. The cells cultured under the high-light show significantly higher photosynthetic CO2 affinity than those grown at the low-light level. It is concluded that the regulations of Caext activity and photosynthetic CO2 affinity are dependent not only on CO2 concentration but also on light availability, and that the development of higher Caext activity and CO2 affinity under higher light level could sufficiently support the photosynthetic demand for CO2 even at low level of CO2.