Plants, aquatic insects, and mosquitofish (Gambusia affinis) 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 26.0 μg/g (dry wt) in water boatman (Corixidae) to 119 μg/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 widgeongrass (Ruppia mari-tima) (1,630 μg/g) was high enough to impair avian reproduction. There were no differences in arsenic concentrations between locations, and concentrations in all food-chain organisms (<1.9 μg/g) were lower than those reported to cause adverse effects in wildlife. Within-location differences were observed for all three contaminants at Kesterson and for selenium at Volta, but there was no consistent pattern to these differences. Between-year comparisons showed that selenium concentrations in mosquitofish generally decreased at Kesterson, but remained about the same at Volta over the 3 years. Selenium concentrations in insects from 1985 were lower at Kesterson than 1983, but were similar to 1984. Concentrations in plants were generally higher in 1983 and lower in 1984 compared with 1985. Boron concentrations in plants were generally higher in 1985, but in mosquitofish and insects, boron concentrations remained about the same all 3 years. Most arsenic concentrations did not change significantly between years.