In the Huizache-Caimanero lagoon system on the Pacific coast of Mexico, data concerning lagoon hydrography was obtained and qualitative studies were made of the lagoon vegetation, bird fauna, pelagic and benthic fauna. Penaeus (Litopenaeus) vannamei Boone and Penaeus (Litopenaeus stylirostlis Stimpson were the dominant penaeids and, numerically, Lile stolifera (Jordan and Gilbert), Anchoa panamensis (Steindachner) and Mugil curema (Civier and Valenciennes) were the dominant fish species within the lagoon. Two species of portunid crab, Callinectes arcuatus Ordway and Callinectes toxotes Ordway, occurred in the lagoon with the former by far the more abundant. Population densities were generally higher in the "esteros" (estuaries) and canals than in the open lagoons. Postlarval penaeid immigration was maximum in the wet season. Subsequent growth was rapid and estimates of growth rate of up to 0.4 mm carapace length/day were obtained for P. vannamei . The natural diets of penaeid shrimps and portunid crabs, and the feeding modes of some important fish and bird species are discussed.