Estuarine/marine fungi tolerated Ni better when grown on a nutrient medium supplemented with seawater than when exposed on a non-marine medium. The ameliorating effect of seawater or salinity on the toxicity of Ni to mycelial proliferation was related to the Mg, rather than to the Na or Cl, ions in the marine systems. This interaction between Mg and Ni was not unique to marine fungi, as Mg also decreased the toxicity of Ni to non-marine fungi. The mechanism whereby seawater reduced Ni toxicity is apparently different from the mechanism whereby seawater reduced the toxicity of Cd and Hg, which has been postulated to be the result of chlorinity and the formation of negatively-charged Cd-Cl and Hg-Cl complexes, rather than a competitive interaction with Mg.