An analysis of the seasonal phytoplankton cycle of the Georges Bank area of the Gulf of Maine (Riley, 1946) has shown that fluctuations in the population are largely controlled by the seasonal variations of a few environmental factors. An equation was developed in which the rate of change of the population in respect to time was equal to the photosynthetic rate minus the respiratory rate minus the rate of removal of the population by grazing. These rates were in turn evaluated in terms of incident solar radiation, transparency of the water, the depth of the mixed surface layer, the quantity of phosphate, temperature, and the amount of zooplankton.