The effects of selenium (Se) addition to leaves (plants) or substrate (plants or fungi) on mercury (Hg) uptake, transfer, ligand environment and toxicity in the plant/fungi-slug food chain were studied. Plants were grown on naturally (HgS, Idrija) or artificially (HgCl2) contaminated substrate, without or with foliarly applied Se. Fungi were grown in liquid media or on agarose plates, with addition of HgCl2 or HgS and with or without added Se. Additionally, fungi (Boletus spp. and Scutiger pes-caprae) were picked from the forest from non-polluted (NP) or Hg-polluted (Idrija) sites. At the end of experiments morphological and biochemical changes were observed as fresh and dry biomass (plant shoots and roots; fungal mycelium; slug hepatopancreas and muscle), concentrations of photosintetic pigments in shoots and concentrations of malondialdehyde (MDA) as a measure of oxidative stress in the observed tissues. Mercury and Se concentrations were measured in freeze-dried and pulverised material. Ligand environment of Hg was studied using synchrotron based X-ray absorption spectroscopy. We showed that Hg bioavailability (BA) for plants and fungi depended on Hg speciation in substrate or growth medium. Selenium biofortification had no effect on Hg BA for plants, while the Se addition increased the BA of Hg for slugs with an opposite physiological response. Feeding on biofortified plants from Idrija soils increased toxicity in slugs, but alleviated it when ingesting HgCl2 exposed biofortified plants. Boletus spp. accumulated significant amounts of Hg, especially the ones from Idrija. Also, S. pes-caprae accumulated significant amounts of Hg at NP. Mercury in examined fungi from NP was mainly bound to di-thiolate and HgSe ligands, while at Idrija site the proportion of HgSe and tetra-thiol ligands increased with increasing Hg contents in the fungi. Experiments with the slugs showed that Hg present in fungi was in bioavailable form as it accumulated in their hepatopancreas and muscle tissues. The BA of Hg increased when fungi, cooked in water or acetic acid, were ingested, but final Hg concentrations in hepatopancreas decreased. The toxic effects of Hg exhibited as reduced growth and increased MDA level in the slug’s hepatopancreas counteracted with increasing Se concentrations in the ingested fungi, pointing to a protective role of Se against Hg toxicity through HgSe complexation.