Using chlorophyll/P700 ratios, the size and number of photosynthetic units were estimated, as a function of light-shade adaptation in two species of marine phytoplankton: Skeletonema costatum, a diatom, and Dunaliella tertiolecta, a chlorophyte. In the diatom, light-shade adaptation is characterized primarily by changes in the size and not the number of P700 units, whereas in the chlorophyte, overall changes in chlorophyll content are related to changes in the number and not the size of P700 units. A correlation between the characteristics of P700 units and photosynthetic responses was not established. Both strategies of light-shade adaptation effectively harvest and transfer light energy to reaction centers, however, the Skeletonema strategy is more effective at subsaturating intensities. The two strategies may represent an evolutionary divergence in photosynthetic adaptation to variations in light intensity.