The effects of simultaneous limitations of light and nutrient on growth were investigated with turbidostat and chemostat cultures of Scenedesmus sp. and Fragilaria crotonensis. The combined effects were greater than the sum of individual effects and were not multiplicative. Under nitrate-limited conditions the cell quota of the limiting nutrient for a constant growth rate and the subsistance quota increased as irradiance decreased. Within a certain limit of growth rates, light and q sub N can compensate for each other in maintaining growth rate. Under nutrient-sufficient conditions, cell quotas of C, N, and P, and cellular chlorophyll a concentration increased as light decreased below saturation. The cell quotas are the maximum level that can be achieved at a given irradiance.