Groundwater in many parts of the world is contaminated with arsenic and nitrate. To date no technology is available that can in situ remediate arsenic and nitrate pollution of groundwater using one single bacterial species. In this study, a novel anaerobic nitrate-dependent Fe(II)-oxidizing bacterium, Clostridium sp. strain PXL2, was isolated from anoxic activated sludge. The strain PXL2 could efficiently oxidize Fe(II) associated with the reduction of nitrate. Scanning Electron Microscope (SEM), X-ray diffraction (XRD) and Fourier Transform Infrared Spectrometry (FTIR) analysis showed that strain PXL2 formed nanosize and poorly crystalline Fe(III) oxides which encrusted the cells, with the viability and function of the cells might being inhibited by the encrustation of Fe(III) oxides. It was found that strain PXL2 was an excellent candidate microbe for bioremediation of nitrate and arsenic pollution. It could effectively and simultaneously remove As(III) and nitrate from water due to its Fe(II) oxidation and denitrtification activity. 40.6-100% of NO3- was removed from an initial concentration of 12.3 mM. High concentrations of As(III) decreased nitrate removal. With initial concentrations of 10 M, 65 M and 125 M As(III), 100% of As(III) was removed in a 20 d treatment. Arsenic was removed by adsorption to the biogenic Fe(III) oxides but not by adsorption to the cells. Strain PXL2 was also able to oxidize As(III) to less mobile and less toxic As(V) and this may significantly enhance the performance of bioremediation of arsenic pollution. Anaerobic nitrate-dependent Fe(II)-oxidizing species, such as strain PXL2, has great application potential for in situ remediation of nitrate and metal(loid)s polluted groundwater with advantages of high efficiency and low cost.