The response of the coccolithophore <i>Emiliania huxleyi</i> to rising CO<sub>2</sub> concentrations is well documented for acclimated cultures where cells are exposed to the CO<sub>2</sub> treatments for several generations prior to the experiment. The exact number of generations required for acclimation to CO<sub>2</sub>-induced changes in seawater carbonate chemistry, however, is unknown. Here we show that <i>Emiliania huxleyi</i>'s short-term response (26 h) after cultures (grown at 500 μatm) were abruptly exposed to changed CO<sub>2</sub> concentrations (~190, 410, 800 and 1500 μatm) is similar to that obtained with acclimated cultures under comparable conditions in earlier studies. Most importantly, from the lower CO<sub>2</sub> levels (190 and 410 μatm) to 750 and 1500 μatm calcification decreased and organic carbon fixation increased within the first 8 to 14 h after exposing the cultures to changes in carbonate chemistry. This suggests that <i>Emiliania huxleyi</i> rapidly alters the rates of essential metabolical processes in response to changes in seawater carbonate chemistry, establishing a new physiological "state" (acclimation) within a matter of hours. If this relatively rapid response applies to other phytoplankton species, it may simplify interpretation of studies with natural communities (e.g. mesocosm studies and ship-board incubations), where often it is not feasible to allow for a pre-conditioning phase before starting experimental incubations.