Selenium oxyanions are toxic to living organisms at excessive levels. The accumulation of selenium in certain environments requires the development of efficient detoxification processes. In this study, we demonstrated the ability of two Anaeromyxobacter dehalogenans strains to readily convert toxic Se(IV) to less toxic insoluble Se(0), as confirmed by energy-dispersive X-ray spectrometry and X-ray photoelectron spectroscopy analyses. A. dehalogenans strains have been recently recognized as a group of versatile bacteria ubiquitously distributed in diverse natural environments best known for their bioremediation capacity to immobilize heavy metals and radionuclides. Discovery of the Se(IV)-reducing capacity by these microorganisms adds selenium detoxification to the bioremediation capacities of A. dehalogenans. It is also shown that the Se(IV)-reducing activity by A. dehalogenan could be inhibited by high levels of Se(IV). With Se(IV) reduction being proposed as a detoxification mechanism in other microorganisms, it is likely that the inhibition of Se(IV) reduction by Se(IV) was a result of the severe toxicity of Se(IV) at elevated concentrations. Further studies are needed to elucidate the mechanisms of Se(IV) reduction and its inhibition by Se(IV) in order to understand the potential roles of these ecologically important microorganisms in the biogeochemical cycling of selenium and the biotransformation of toxic forms of selenium. (C) 2009 Elsevier Ltd. All rights reserved.