Plants, aquatic insects, and mosquitofish were collected from Kesterson Reservoir, Merced County, California, and a nearby reference site (Volta Wildlife Area) to compare concentrations of three contaminants found in 1985 with those reported in 1983 and 1984. Mean selenium concentrations in food-chain organisms from sites at Kesterson in 1985 ranged from 27.0 ug/g dry weight in water boatman to 119 ug/g in mosquitofish. All mean selenium concentrations at Kesterson were significantly higher than those from Volta and were sufficient to have caused the impaired avian reproduction observed at Kesterson. Boron concentrations were also significantly higher at Kesterson, and at one pond, the mean concentration in sidgeongrass was high enough to impair avian reproduction. Arsenic, selenium, and boron were also considered.