Despite extensive documentation of mercury (Hg) bioaccumulation and its neurotoxic effects on biota in aquatic ecosystems, little is known about Hg dynamics in terrestrial ecosystems. High elevation areas of the northeastern United States have been identified as particularly vulnerable terrestrial sites to atmospheric Hg accumulation, but empirical data are lacking. The goal of this study was to measure Hg accumulation in multiple biological compartments along a continuously forested altitudinal gradient, largely unimpacted by human development. Mercury concentrations were measured in foliage, litterfall, soils, and in forest-floor inhabiting organisms. Catharus thrushes and red-backed salamanders (Plethodon cinereus) spanned the altitudinal gradient of this study and served as habitat-specific bioindicator taxa. Mercury concentrations were found to increase with elevation in the three uppermost forest floor layers, and in foliage. The Hg concentration of hardwood litterfall increased over the growing season as did concentrations in the foliage of three hardwood species. Mercury concentrations in softwood foliage and litterfall did not increase within the time frame of the growing season, but Hg concentrations in foliage increased with needle age-class. Mercury concentrations in thrushes and red-backed salamanders reflected the background Hg in their immediate environment and increased with elevation. Blood Hg concentrations declined in all thrushes during the breeding season, possibly as a reflection of a changing diet. It is also possible that thrushes arrived with Hg burdens carried over from their migratory wintering grounds that subsequently decreased as they came to steady-state with respect to the breeding ground environment. Investigations into the diet, territoriality, social system, and physical condition of Bicknell's thrush (Catharus bicknelli) wintering in the Dominican Republic are presented, along with documentation of wintering individuals with the highest blood Hg concentrations in this study. Together these results show that environment-wide Hg concentrations increase with altitude in terrestrial ecosystems of the northeastern United States and that Catharus thrushes and red-backed salamanders are sensitive bioindicators of these changing concentrations. Biota of high elevation terrestrial ecosystems may be at greater risk of Hg contamination, both in the United States and in other less-studied areas of the developing world where Hg emissions are increasing.