Marine unicellular calcifying algae, coccolithophorids, strongly interact with the global climate and carbon cycle through their photosynthetic and calcite formation activities. As their distribution has been known to be common in high latitudes in the ocean, their response to low temperature may be unique to support growth and distribution. This study intends to elucidate the morphological and physiological responses of the two species of coccolith-producing haptophytes namely, Emiliania huxleyi and Gephyrocapsa oceanica, to temperature. Enlargement of cell volume and its calcification are enhanced while cell growth is greatly suppressed at low temperature. Further, coccolithophorid E. huxleyi increase their production and unsaturation of alkenones by stimulating their chain elongation and unsaturation reactions when the cells are stressed by low temperature. These changes are reversibly regulated at high temperature. The relationship among these phenomena is not yet clearly elucidated, but the results suggest that they may be closely associated under regulation by temperature-sensitive signal transduction system.