A preliminary ecosystem trophic structure model was built for Monterey Bay, California, USA, and analyzed using the ECOPATH II program. The model has fifteen living boxes plus one box for detritus. Three different primary production values were used to evaluate the model. They correspond to: (1) winter low, (2) mean high upwelling and (3) occasional very high upwelling production observed in the bay. Biomass, ecotrophic efficiencies, flows of detritus and respiration/assimilation estimated by the model were in agreement for values measured in the bay and/or similar environments. This suggests that even though the model is in a preliminary stage, it possesses characteristics of the natural system.