Acidic wastewaters from industrial and mining activities constitute a wordwide environmental hazard. 'Acid mine drainage' (AMD) waters are often highly acidic (pH<4), contain elevated concentrations of sulfate and dissolved metals, and are toxic to most life forms. Whilst bioremediation of acidic, metal-contaminated waters using sulfate-reducing bacteria (SRB) has been successfully demonstrated in pilot-scale and full-scale operations during the past decade, the sensitivity of characterised SRB to acidity limits their use in AMD remediation. In the current study, we have evaluated the potential use of novel acidophilic SRB for remediating acidic waste waters, in comparision with, and in conjuction with, neutrophilic SRB. Three SRB-populations (a mixed population of acidophilic isolates, a neutrophilic culture and a mixed acidophilic/neutrophilic consortium) were immobilised on porous glass beads, packed into perspex columns and the three bioreactors percolated with synthetic medium for several months. Energy and carbon source utilisation, and tolerance to acid stress of the different consortia were evaluated. Acidophilic SRB were more efficient than the neutrophilic culture in coupling ethanol oxidation to sulfate reduction and all of the substrates tested were oxidised to acetic acid. The bioreactors containing acidophilic SRB reduced sulfate and generated alkalinity when challenged with influent at pH 3 and above, indicating that such bacteria have potential for bioremediating highly acidic waste waters. Average conversion rates of 0.25-0.30 g SO42- reduced dm(-3) day(-1) were achieved with bioreactors containing acidophilic SRB, percolated with a pH 4 liquid medium.