Kesterson Reservoir (Kesterson) received subsurface agricultural drainwater containing high levels of salts and other minerals from farmland in the San Joaquin Valley of California. Aquatic plants and invertebrates were sampled at Kesterson in May, August, and December of 1984. The reservoir supported a different biota and lower species diversity than a nearby control site (Volta WMA). Kesterson had a greater plant and seed biomass while Volta had a greater invertebrate abundance. Submergent habitat at Kesterson was dominated by widgeongrass (Ruppia maritima) while Volta was dominated by horned pondweed (Zannichellia palustris). Several aquatic invertebrates, including Amphipoda, Exlais, Gastropoda, Neomysis, Hirundinea, and Belostomatidae were common at Volta but were never observed at Kesterson. Kesterson supported a greater abundance of diatoms (Nitzschia), Oligochaeta, Ephydridae, Stratiomyidae, Tabanidae, and Syrphidae while these were rarely encountered at Volta. Community structure at Kesterson was most likely influenced by high concentrations of salts, nitrogen, boron, and possibly selenium. Bioaccumulation of selenium and other trace elements in wetlands and waterfowl foods at Kesterson was investigated during May, August, and December of 1984. High concentrations of selenium were found in water, sediments, terrestrial and aquatic vegetation, and aquatic insects. Selenium concentrations in aquatic plants and insects ranged from 2 to 310 ppm and were about 10 to 290 times those found at Volta. Concentrations in waterfowl food plants and insects at Kesterson were as high as 64 times greater than those reported to be a health hazard to birds. Seasonal variations in selenium concentrations were observed in some plants, but few consistent seasonal patterns were observed in aquatic insects, and few differences in selenium accumulation were found among ponds. Distribution of selenium in plant parts was not uniform during a growing season, as rhizomes contained higher concentrations than seeds. Most biota bioaccumulated selenium to levels greater than 1000 times the concentration in water, some nearly 5000 times. Mean concentrations of boron in aquatic plants and insects were usually 2 to 52 times those at Volta. Concentrations of other trace elements (i.e. arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, and nickel) at Kesterson were too low to be toxic to biota.