During exponential growth in batch culture, assimilation numbers of eleven algal species ranged from 1.6–20.8, with a mean value of 5.3 g C/g Chlorophyll a/hr. The highest assimilation number of 20.8 g C/g Chlorophyll a/hr was observed in Coccolithus pelagicus, due to the relatively low concentration of chlorophyll a/cell. The assimilation number declined from exponential to stationary phase in batch cultures for ten algal species, but increased with age in batch culture in Amphiprora paludasa (a benthic diatom). The assimilation number declined with decreasing growth rate in nitrate-limited chemostat cultures of Phaeodactylum tricornutum and in iron-limited chemostat cultures of Phaeodactylum tricornutum and Isochrysis galbana.