Los Petenes Biosphere Reserve (LPBR), in Campeche, Mexico, is a coastal strip that integrates a land portion -where petenes are distinguished as unique ecosystems-, and a marine portion that highlights the presence of mangroves and seagrasses as key ecosystems. In LPBR, seagrasses have been identified as the largest and best conserved ecosystem in the Gulf of Mexico. There is a diverse biotic community associated with seagrasses, where fish are the most abundant macrofauna. The objective of this research is to describe the spatial and temporal behavior of the biomass of seagrasses and fish in the littoral portion of the reserve. A database of temperature and precipitation from three meteorological stations was built to describe the regional climatic behavior. Four sampling campaigns were conducted, visiting 12 previously located sites. At each site, environmental variables were measured, and samples of seagrasses were gathered by free immersion; also, fish samples were taken with an experimental trawl. The general weather in the reserve has two defined seasons (rainy and dry). This weather regulates the environmental variables registered in the sampling sites, and it is possible to identify associations between the abundance of both grasses and fish. Grass biomass reaches maximum values of 664.9 g/m(2), and fish biomass 1.61 g/m(2). We identified 12 dominant fish among which Lagodon rhomboides and Haemulon plumierii stand out. The high productivity of seagrasses in LPBR is confirmed, in relation with climate seasonality and the optimization of the environmental conditions of the area.