Colonies of iron-oxidising acidophilic bacteria were isolated on solid media containing up to 500 mM NaCl from non-saline samples from the Rio Tinto (Spain). One of these isolates was identified as an "Acidithiobacillus ferriphilus" strain. Laboratory cultures of the type strain of Acidithiobacillus ferridurans grown on hydrogen for one year were also found to adapt to the presence of 500 mM salt. This culture also grew on sulfur, but not on ferrous iron, in media containing 500 mM NaCl. It regained its ability to oxidise iron only after protracted incubation in salt-free media. Molecular analysis found an insertion of about 1.5 kb in the regulatory region of the rus operon of the strain that was unable to oxidize iron. This insertion was lost in the strain that had subsequently regained this ability. Our results suggest that chloride tolerance and dissimilatory oxidation of iron by these bacteria do appear to be mutually exclusive.