We studied the effect of zooplankton diet on MeHg accumulation in different zooplankton size-fractions from lakes of different trophic status. Using fatty acid biomarkers, we tested the hypotheses that (a) variations of MeHg concentrations are determined bythe taxonomic composition of zooplankton and (b) concentrations of dietary algal and bacterial compounds can predict MeHg concentrations of seston (10-64 microm), micro- (100-200 microm), meso- (200-500 microm), and macrozooplankton (>500 microm) in lakes on Vancouver Island, Canada. MeHg concentrations increased from seston (4-48 ng g dry weight(-1)) to macrozooplankton (94-240 ng g dry weight(-1)), indicating that MeHg accumulated as a function of plankton size. Results from linear regression analysis showed that MeHg concentrations were not significantly related to the taxonomic composition of zooplankton. However, using dietary lipid biomarkers, we demonstrated that bacterial diet (R2 = 0.50; p < 0.01) could better predict variations of MeHg concentrations in zooplankton than essential algal diet (R2 = 0.35; p < 0.01). Because MeHg accumulation within the planktonic food web was higher (20x) than the observed accumulation of total bacterial (6.5x) and algal (4.7x) diet biomarkers, zooplankton retained dietary MeHg more efficiently than bacterial and algal diet compounds. These results indicate that MeHg of macrozooplankton, the preferred prey size of planktivorous fish, is more efficiently transferred than essential diet compounds to organisms at higher trophic levels.