The microbial consortium used in continuous-flow,stirred tank processes to treat gold-bearing arsenopyrite concentrates became adapted to high concentrations of arsenic over a number of years.The dominant microorganisms,Acidithiobacillus caldus and Leptospirillum ferriphilum,were found to contain two sets of arsenic resistance genes.One set of ars genes was present in all isolates of a species irrespective of whether they were highly arsenic resistant or not.A second set of ars genes was present on Tn21-like transposons and was found in all strains tested that had been adapted to high concentrations of arsenic.The arsenic resistance transposons present in At.caldus and L.ferriphilum were closely related,but sufficiently different for them to have been acquired independently rather than having been passed from one bacterium to the other.The transposons were transpositionally active in Escherchia coli and were shown to confer higher levels of arsenic resistance than the chromosomally-located ars genes where it was possible to test this.Transposons containing arsenic resistance genes that were identical or closely related to the transposon from L.ferriphilum,originally found in South Africa,were also found in both L.ferrooxidans and L.ferriphilum isolates from South America and Europe.An arsB gene knockout of At.caldus was produced by homologous recombination that demonstrated both the ability of the chromosomal ars genes to confer low levels of arsenic resistance in At.caldus and the development of a genetic system for the creation of knock-out mutants.