Ecto- and endomycorrhizal symbiosis can play a crucial role in protecting plant roots from heavy metals (HMs). The efficiency of protection, however, differs between distinct isolates of mycorrhizal fungi and different HMs. Fungal ecotypes from HM-contaminated sites seem to be more tolerant to HMs than reference strains from non-contaminated sites. The abundance of the extramatrical mycelium was shown to he important for HM binding by the fungus. Most of the HMs were demonstrated to be bound to cell wall components like chitin, cellulose. cellulose derivatives and mela-nins. The chemical nature of HM-binding substances in the fungal cells is not clear. Polyphosphate granules, which were proposed to have this function, seem to be artifacts of specimen preparation. The high N and S concentrations associated with the polyphosphate granules rather indicate the occurrence of HM-thiolate hinding by metallothionein-like peptides.