Specific rates of photosynthesis at saturating irradiances (Popt) by laboratory populations of Chlorella and Anabaena depend on intracellular phosphorus (Q). A hyperbolic model of the form Popt =Popts (1−KQ/Q) is used to describe this relationship for three measurements of biomass (cell number, cell volume, and chlorophyll a). Chlorophyll a provides the most adequate fit to the model. When chlorophyll a is used as the measure of population size, both the maximal specific rate of photosynthesis (Popts) and the subsistence cell quota (KQ) for Anabaena are much larger than for Chlorella. The model provides a conceptual basis for understanding the relationship between rates of photosynthesis and intracellular nutrients in populations of planktonic algae.