The photobiological and ecological derivations of the assimilation number concept are explored. Based on theoretical calculations, a maximum assimilation number of about 25 μg C · hr−1 is calculated. This number is estimated from photosynthetic turnover times and the number of photosynthetic units. The variations in this number are primarily due to the effects of nutrients, temperature, cell size and light history. The interactions of these variables is discussed with emphasis on light history and the physiological adaptability of marine phytoplankton. An equation is presented to estimate light-utilization indices for photosynthesis in the euphotic zone. This index is estimated as 0.43 g C · g chlorophyll−1 · ein−1 · m−2 in New York Bight.