A computer model simulating the seasonal variations, of mixed layer nutrient concentrations, phytoplankton biomass carbon, and herbivorous zooplankton biomass carbon was developed. The simulation was generated using an annual cycle of four environmental parameters: (1) incident solar radiation, (2) upwelling velocity, (3) mixed layer depth, and (4) mixed layer temperature. Simulation results were compared with nutrient and zooplankton biomass data collected on a series of seven cruises made in central Monterey Bay from February through December, 1974. Both observed and simulation zooplankton stocks were characterized by two distinct maxima. The initial peak (1.05 gC/sq m) occurred in late July and was followed by a decline in populations through the month of August. During the fall and early winter, zooplankton biomass increased rapidly to an overall maximum of 1.85 gC/sq m. Individual environmental parameters were tested to ascertain their importance in controlling simulation results. Phytoplankton stocks were influenced principally by changes in incident radiation, whereas temperature variations produced the most significant fluctuations in zooplankton biomass.