A new regulatory model can describe acclimation of phytoplankton growth rate (p), chlorophyll a : carbon ratio and nitrogen: carbon ratio to irradiance, temperature and nutrient availability. The model uses three indices of phytoplankton biomass-phytoplankton carbon (C), phytoplankton nitrogen (N), and chlorophyll a (Chl). The model links the light-saturated rate of photosynthesis to N: C, requires that Chl a synthesis be coupled to nitrogen assimilation, and includes several regulatory features. These include feedback inhibition of the nitrogen assimilation rate by increases in the N: C ratio, as well as regulation of Chl a synthesis by the balance between light absorption and photosynthetic carbon fixation. The model treats respiration as the sum of the maintenance metabolic requirement and the cost of biosynthesis. In addition, the model can account for accumulation and mobilization of energy reserves (i.e. variability of N: C) and photoacclimation (i.e. variability of Chl : N and Chl : C) in response to variations in irradiance and nutrient availability. The assumptions of the model are shown to be in agreement with experimental observations and the model output compares favorably with data for cultures in balanced and unbalanced growth.