We compared the effect of CO2 concentration ([CO2], ranging from ∼ 5 to ∼ 34 μmol -1) at four different photon flux densities (PFD = 15, 30, 80 and 150 μmol m-2 s-1) and two light/dark (L/D) cycles (16/8 and 24/0 h) on the coccolithophore Emiliania huxleyi. With increasing [CO2], a decrease in the particulate inorganic carbon to particulate organic carbon (PIC/POC) ratio was observed at all light intensities and L/D cycles tested. The individual response in cellular PIC and POC to [CO2] depended strongly on the PFD. POC production increased with rising [CO2], irrespective of the light intensity, and PIC production decreased with increasing [CO2] at a PFD of 150 μmol m-2 s-1, whereas below this light level it was unaffected by [CO2]. Cell growth rate decreased with decreasing PFD, but was largely independent of ambient [CO2]. The diurnal variation in PIC and POC content, monitored over a 38-h period (16/8 h L/D, PFD = 150 μmol m-2 s-1), exceeded the difference in carbon content between cells grown at high (∼ 29 μmol -1) and low (∼ 4 μmol -1) [CO2]. However, consistent with the results described above, cellular POC content was higher and PIC content lower at high [CO2], compared to the values at low [CO2], and the offset was observed throughout the day. It is suggested that the observed sensitivity of POC production for ambient [CO2] may be of importance in regulating species-specific primary production and species composition. © 2002 Elsevier Science B.V. All rights reserved.