A preliminary quantitative model of the trophic structure in Tampamachoco lagoon was obtained using Ecopath II and data from relevant studies to date in the area. This is a detritus-based ecosystem. The most important first-level consumers are meiofauna, with an energy flow of 57.8 g m(-2) year(-1) of dry weight (dw), followed by zooplankton with a flow of 15.7, white mullet with 9.5, and oysters with 7.5 g m(-2) year(-1) dw. Meiofauna acts as the principal link to higher trophic levels. White mullet and oyster represent most of the biomass (92%) that is subject to commercial fishery. Crustaceans are the most important secondary consumer group with a biomass how of 12.8 g m(-2) year(-1) dw. Shrimp, anchovy, and goby are frequent prey of the principal predators in the system. Catfish, snook, croaker, and portunid crabs are the most important third and fourth level consumers. Commercial fishery plays a secondary consumer role with a mean trophic level of 3.08 and with efficiency relative to the primary productivity of 0.0087, which is high compared to other tropical fisheries based on top predators. (C) 1998 Elsevier Science B.V. All rights reserved.