Bacterial reduction of selenium (Se) oxyanions (Se[VI] and Se[IV]) to elemental Se (Se[0]) is one of the major biogeochemical processes removing Se from agricultural drainage water and depositing Se in the sediment. This study was conducted to characterize Se-reducing bacterial populations in Lost Hills evaporation pond sediment and to observe their response to Se(VI) and organic C amendments. Se(VI) was removed from the dissolved phase in the sediment slurries amended with organic C with a decrease in redox potential (E-h). Se(VI) concentrations decreased from 2137 to 79 mu g L-1 after 9 days of incubation in a 5% soil slurry. Upon our screening process, 9 Se(VI)-and 14 Se(IV)-reducing bacteria were isolated from sediment slurries and identified by amplification and sequencing of 16S rDNA. Bacillus strains appeared to be dominant in the bacterial assemblages active in Se(VI) and Se(IV) reduction in the sediment. Halomonas pacifica and Staphylococcus warneri were also identified as Se(IV)-reducers. Indigenous bacteria have a significant role in the biogeochemical cycling of Se and may be stimulated by addition of a suitable organic source for Se reduction. The bacterial strains isolated from salt-affected and Se-contaminated Lost Hills evaporation pond sediment may have potential application in removing Se from high salt drainage water. (c) 2005 Elsevier Ltd. All rights reserved.