Photosynthesis utilizes solar radiation, and the intensity and spectrum vary largely with time and space. Phytoplankton can adapt to such light changes by varying the physiological characteristics such as cellular light absorption and pigment composition. Study on those adaptation is required to gain a better understanding of a biogeochemical cycle in the marine ecosystem. In this research, the photoadaptive responses of marine phytoplankton were assessed, focusing on cellular light absorption and pigment composition. Chlorophyll a (Chl a) specific absorption coefficient of phytoplankton is the fundamental index of cellular light absorption. Chl a specific absorption coefficient at six irradiances ranging from 25 to 750 micro-mol m^-2 s^-1 was quantified for six species of phytoplankton with varying size. Chl a specific absorption at 440 nm and 675 nm increased logarithmically with irradiance for all species. For the six irradiances, significant relationships were obtained between cell volume and Chl a specific absorption at 675 nm, which decreased significantly with increasing cell volume at all irradiances. At higher irradiance than 190 micro-mol m^-2 s^-1, however, size-dependence of absorption at 440 nm weakened due to the effects of absorption by photoprotective pigments. In high light conditions, if phytoplankton cells are unable to utilize the high level of energy absorbed by pigments, the excess energy will cause damages to intracellular materials or metabolic processes. The xanthophyll pigments such as diadinoxanthin (DD) and diatoxanthin (DT) play a role in protecting the cell itself against such photoinhibition. The contribution of xanthophyll pigment, DD plus DT, absorption to total in vivo pigment absorption was examined in three marine diatoms grown at 6 irradiances between 90 and 750 micro-mol m^-2 s^-1. As the growth irradiance varied from 90 to 750 micro-mol m^-2 s^-1, the relative contribution of absorption by DD plus DT to total in vivo pigment absorption (for wavelengths from 400 to 700 nm [PAR]) increased from 4.5 to 17 % for Phaeodactylum tricornutum, 5.8 to 19 % for Chaetoceros gracilis and 13 to 30 % for Thalassiosira weissflogii. For the purpose of evaluating the photoprotective function of xanthophyll pigments in the natural environment, the relationship between xanthophyll pigments and irradiance was examined during the phytoplankton blooms dominated by diatoms and dinoflagellates in Sagami Bay from the end of April to July 2000. Daily irradiance was variable and ranged approximately from 20 to 50 mol m^-2 day^-1. The values of (DD+DT)/Chl a, used as indicator of a photoprotection by xanthophyll pigments, ranged from 0.0053 to 0.18. In case of active phytoplankton assemblages with higher Chl a concentration than 2 mg m^-3, a significant linear correlation was found between irradiance of the previous day and (DD+DT)/Chl a. This result may indicate that the xanthophyll pigments acted as a photoprotection against excess light energy in natural phytoplankton community. The results of this research will contribute much to the study on ecological strategies of phytoplankton which are the dominant primary producers in the marine ecosystem.