Mercury (Hg) concentrations in seston (biotic and abiotic particles < 200 microm) were measured during summer thermal stratification in nine lakes in the Adirondack Mountains of New York State that exhibit a range of physical and chemical characteristics. Fractionation of Hg bound to particulate matter was conducted to identify the influences of seston density and water chemistry on Hg accumulation among various ecologically important seston size classes (0.2-2, 2-20, 20-200, and > 153 microm). Total dissolved Al (< 0.45 microm) in lake water was significantly related to Hg concentrations in the 20-200 microm size class, and was positively correlated with Hg concentrations in the other seston size classes. Seston density was negatively correlated with seston Hg concentrations and significantly related to Hg bound by seston in the 0.2-2 microm size class. The results suggest that surface water Al concentrations and seston density influence Hg accumulation at the base of the aquatic food chain.