SUMMARYThe relationship between caleite production and dissolved inorganic carbon (DIC) utilization was investigated using high-and low-calcifying strains of Emiliania huxleyi aerated with either air (0-03 % v/v CO^) or COj-free air. With cultures grown on air, calcite production was eight-fold greater in the high-calcifying eulture, but growth rates for high-and low-calcifying cultures were similar. Growth and caleite production were accompanied by a concomitant decrease in DIC and free CO^ in the high-calcifying culture, showing that HCO3" provides inorganic carbon for calcite synthesis. In low-calcifying cultures DIC and free CO.^ were relatively constant, confirming that cells acquire inorganic carbon mainly by the diffusive entry of free CO,.When cultures were aerated with CO^-free air the free CO^ concentration was below tbe /^^^[COJ for high-and low-calcifying cells and the low-calcifying cells were unable to grow. Growth of high-calcifying cells was observed, and the calcite yield was little changed from cultures gassed with air (003 °o v/v CO^). At tbese minimal COj eoncentrations HCO3" may provide inorganic carbon for calcite synthesis and CO^ for photosynthesis, allowing growth to oeeur. Calcite synthesis by E. huxleyi decreases DIC and carbonate alkalinity in cultures not in equilibrium with the gas phase.