Abstract: Selenium is an essential trace element for humans and animals,involved in diverse physiological actions.Thus,different forms of selenium have been developed for future application.To evaluate the toxicity of nano-selenium on zebrafish(Danio rerio)embryos,the light and electron microscopic techniques were used in the present study with two different forms of selenium including DL-selenomethionine and nano-selenium.According to the laser particle size analyzer and electron microscope,the size distribution of nano-selenium particle was uniform.In addition,an average particle size of nano-selenium was about 25 nm.The results showed that the values of median lethal concentration(LC₅₀)of nano-selenium at 96 hours post-fertilization(hpf)and 120 hpf,were 7.18 and 5.68 μmol/L,respectively.However,as for the treatment of DL-selenomethionine,the corresponding values of LC₅₀ at 96 and 120 hpf,were 67.32 and 54.66 μmol/L,respectively.In addition,most of zebrafish embryos inflated the swim bladder successfully at 120 hpf in the groups treated without selenium.However,as for the zebrafish embryos treated with selenium at certain concentrations,abnormal phenotypes were observed in the present study such as axial malformation(AM),pericardial edema(PE),yolk-sac edema(YSE)and uninflate the swim bladder(USB).Parts of zebrafish embryos were even found dead before and/or after hatching.Through the statistical analysis of the data,the median teratogenic concentration(TC₅₀)of nano-selenium and DL-selenomethionine was 3.27 and 44.22 μmol/L,respectively at 120 hpf.It indicated that the toxicity of nano-selenium was significantly higher than that of DL-selenomethionine when treated indirectly on zebrafish embryos.The related mechanism should be studied in the future research.The present study was to present the related basis foundation and information for the safe application of nano-selenium particles and extend the corresponding toxicology theory for aquatic animals.