Mercury (Hg) can be released into the environment by various natural and industrial processes. Given the potential for environmental discharges from a number of sources and the severity of hazards associated with this highly toxic metal, potential mercury transformations must be well understood to effectively predict and prevent harmful human and environmental health effects. Bioindicators play an important role in identifying the factors controlling Hg toxicity and bioavailability and can ultimately be used to evaluate hazardous situations. A methodology using the earthworm Eisenia foetida has been developed to assess Hg bioavailability in mine tailings and aqueous solutions. Results indicate that E. foetida accumulate Hg and a positive correlation exists between Hg concentrations in worm tissues, the substrate they consume and length of exposure. To investigate the effect of natural organic acids on Hg bioavailability, metallic Hg (Hg0) was dissolved in tannic acid and ‘fed’ to the worms in a substrate of paper and silica sand. Total Hg and methylmercury (MeHg) were analysed to determine whether methylation of Hg was occurring in the substrate, directly within worm intestines, or in the tannic acid–Hg solution. The MeHg:total Hg ratio was up to 160 times higher in worm tissues than both the tannic acid–Hg solution and the substrate. This result is particularly significant in organic-rich systems, where naturally occurring organic acids may be facilitating methylation within organisms digestive tracts.