Selenium (Se) is an essential trace element in living systems.Microorganisms play a pivotal role in the selenium cycle both in life and in environment.Different bacterial strains are able to reduce Se(Ⅳ) (selenite) and (or) Se(Ⅵ) (selenate) to less toxic Se(0) with the formation of Se nanoparticles (SeNPs).The biogenic SeNPs have exhibited good application prospects in medicine,biosensors and environmental remediation.These microorganisms might be explored as potential biofactories for synthesis of metal(loid) nanoparticles.A strictly aerobic,branched actinomycete strain,ES2-5,was isolated from a selenium mining soil in southeast China,identified as Streptomyces sp.based on 16S rRNA gene sequence and morphologic characteristics.Strain ES2-5 was resistant to some heavy metal(loid)s such as Se(Ⅳ),Cr(Ⅵ) and Zn(Ⅱ) with minimal inhibitory concentration (MIC) of 50,80 and 1.5 mM,respectively.Both SEM and TEM-EDX analysis showed that Se(Ⅳ) was reduced to Se(0) with the formation of SeNPs as a linear chain in the cytoplasm.The sizes of the SeNPs were in the range of 50-500 nm.The SeNPs are released via hypha lysis or fragmentation.The cellular concentration of glutathione per biomass decreased along with Se(Ⅳ) reduction,and SeNPs were not observed in different sub-cellular fractions in presence of NADPH or NADH as an electron donor,indicating glutathione is most possibly involved in Se(Ⅳ) reduction.It would be very useful in medicine such as antioxidant agent,as well as Se bioremediation.