Emiliania huxleyi and Gephyrocapsa oceanica are the predominant coccolithophorid species that produce blooms in the ocean and affect the global environment. These species are capable of carbon fixation by both photosynthesis for organic matter production and by intracellular calcification for coccolith production. Both processes were strongly affected by the nutrient status in a laboratory culture. The coccolith production was stimulated by the addition of a high concentration of sodium bicarbonate and by the depletion of phosphate. Interestingly, when the calcification was stimulated, the increase in cell number during algal growth was greatly suppressed and then the cell volume increased. When the growth rate was increased under nutrient-sufficient conditions, the cells became very small in size and most of them bore few or no coccoliths. The data from laboratory experiments show that the cell growth and calcification proceeded apparently independently at different phases. We, therefore, assume that the coccolithophorid blooms in the ocean might be separated into two phases; firstly, the increase in cell population might be triggered by an adequate supply of nutrients to enhance algal growth and then the calcification might subsequently be stimulated when the nutrients become depleted by substantial algal growth.