Chlorella vulgaris,an unicellular microalgae,displays high growth rate under autotrophic and heterotrophic conditions.In addition,C.vulgaris has been proved to possess a good tolerance to high levels of Se based on the previous study,which indicates that the alga is a good hatcher for organic selenium conversion.However,few researches on the biological effects such as the alga growth,the Se conversion,the intracellular enzymes and the species of organic selenium of C.vulgaris exposed to Se were reported.Hence,some biological effects of the alga after exposure to Se(Ⅳ)were investigated in this study. C.vulgaris was cultured and exposed to selenium(Na₂SeO₃)at different concentrations(0,5,10,20,30 mg/L).For investigating the influences of Se on the algal growth and the biochemical effects,the biomass,the intracellular selenium contents,the activities of glutathione peroxidase(GSH-Px)and superoxide dismutase(SOD),and the Se species in cells were assayed,respectively.The results showed that high level(30 mg/L)of Se would inhibit the growth or even facilitate the death of the alga.The total intracellular selenium contents,and the activities of GSH-Px increased with the increase of the selenium concentrations.For example,the activities of intracellular enzymes under the stress of Se at 5,10,20,30 mg/L were 5.59,6.98,11.32,11.52 times compared with those of the control groups.But the activities of SOD increased only when the concentration of stressed Se was more than 20 mg/L.Three main species of intracellular selenium,analyzed with a hyphenated method of SE-HPLC-ICP-MS,were found in C.vulgaris exposed to Se.About 16% of organic selenium was found to be converted in C.vulgariswhich is hopeful to be used as a good biohatcher for the conversion of organic selenium.