This study tests the hypothesis that differences in the densities of plankton across lakes lead to significantvariation in Hg accumulation by the biota. The study comprised (i) a field program to measure and compare total Hg concentrations in phytoplankton (0.4-45 microm), two size classes of zooplankton (45-202 and >202 microm), and fish to phytoplankton and zooplankton densities from 20 lakes in the northeastern United States and (ii) a statistical analysis of fish Hg burdens and plankton densities from 38 additional EMAP lakes in the northeastern United States. We found that phytoplankton density was negatively correlated with Hg concentrations within both the phytoplankton and their primary consumers (small zooplankton). We also found a negative correlation between zooplankton density and Hg concentrations in zooplankton and in both herbivorous and predatory fish. Zooplankton density alone explained more than 40% of the variation in predatory fish Hg levels across lakes. We discuss several mechanisms to explain the negative relationship of plankton density to Hg concentrations in lower trophic levels and to the biomagnification of Hg to fish. Our results suggest that measures of plankton density, which are relatively easy and inexpensive to estimate, may be effective predictors of Hg biomagnification potential across lakes.