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Concentrations of total mercury (Hg) and methylmercury (MeHg) were determined in water, sediment, periphyton, spiders, and amphibians from the streams and desert marsh downstream from Arivaca Lake, Arizona, to better understand their distribution and bioaccumulation. Mean concentrations of MeHg in water ranged from 0.09 to 0.93 ng/L, and mean concentrations of total Hg in sediment ranged from 10.4 to 126 mu g/kg. Hg and MeHg in water and sediments downstream from Arivaca Lake were low enough that they did not exceed human health or ecological thresholds. Hg and MeHg between sites ranged from 0.11 to 1.90 mu g/g Hg and 0.01 to 0.3 mu g/g MeHg in periphyton, from 0.09 to 0.25 mu g/g Hg and 0.04 to 0.10 mu g/g MeHg in spiders, and from 0.15 to 0.38 mu g/g Hg and 0.14 to 0.35 mu g/g MeHg in adult bullfrogs. No Hg toxicity data exist for periphyton or spiders, but MeHg concentrations in tadpoles (0.04 +/- A 0.005 mu g/g) were lower than those known to cause sublethal effects and subchronic mortality. The mean total Hg concentration in adult bullfrogs in the present study was 0.24 mu g/g, which is slightly lower than the mean (0.37 mu g/g) from an Hg-contaminated wetland in California. MeHg bioaccumulated at each successive trophic level, and MeHg bioconcentration factors from the Arivaca watershed were similar to those for periphyton but greater than amphibians in other studies. Local resource managers can use these data to determine if water should be released from Arivaca Lake to recharge the aquifer downstream or to decrease Hg methylation in the reservoir.