The effects of 13 indigenous microorganisms with distinct colony forms isolated from soils and sediments containing W of 64~113 mg/kg on the behavior of dissolved W(VI) were experimentally investigated under anaerobic condition. Compared with abiotic control (94 mg/L), eight bacteria decreased more than twice the amount of dissolved W (192~378 mg/L) forming precipitates in response to 1,000 mg/L W(VI) supply. The consecutive experiments using Klebsiella sp. and Pantoea sp. among the isolated microorganisms also showed that they reduced greater amounts (210~309 mg/L) of W(VI) to W(IV) than abiotic control. Such microbial reduction of W(VI) might be due to pH decrease by fermentation. In addition, for Klebsiella sp., detoxification was likely one of the mechanisms of W(VI) reduction. The results, which are the first evidence to our knowledge of the effects of microorganisms on geochemical behavior of W, indicate that microorganisms may influence the cycle of W in geological media.