Large single phytoplankton cells are likely to catabolize a smaller fraction of their biomass than do small phytoplankton cells. Under certain environmental conditions, this characteristic may be highly important in regulating the growth rates of large versus small phytoplankton. A model is developed incorporating experimental data on phytoplankton growth, respiration, and sinking rates as a function of cell size. Application of the model to specific oceanic regions shows that the net growth rate of large cells may exceed that of small cells when the mixed layer is relatively deep, despite the larger gross production rates characteristics of the smaller cells.