Moderately thermophilic, iron-oxidizing acidophiles were enriched from coal collected from an open-cut mine in Collie, Western Australia. Iron-oxidizers were enriched in fluidized-bed reactors (FBR) at 60degreesC and 70degreesC; and iron-oxidation rates were determined. Ferrous iron oxidation by the microbiota in the original coal material was inhibited above 63degreesC. In addition to four iron-oxidizers, closely related to Sulfobacillus spp that had been earlier isolated from the 60degreesC FBR, one heterotroph closely related to Alicyclobacillus spp was isolated. The Alicyclobacillus sp. isolated from the Collie coal mine tolerated a lower pH than known Alicyclobacillus spp and therefore may represent a new species. The optimum temperature for growth of the iron-oxidizing strains was approximately 50degreesC and their maximum temperatures were approximately 60degreesC. The FBR was adjusted to operate at 50degreesC and was inoculated with all of the isolated iron-oxidizing strains. At 60degreesC, an iron-oxidation rate of 0.5 g Fe2+ l(-1) h(-1) was obtained. At 50degreesC, the iron-oxidation rate was only 0.3 g Fe2+ l(-1) h(-1). These rates compare favourably with the iron-oxidation rate of Acidianus brierleyi in shake-flasks, but are considerably lower than mesophilic iron-oxidation rates.