A new procedure is developed for the photometric determination of trace selenium in aqueous solutions. The selection of 2-(p-nitrophenyl)-3,5- diphenyltetrazolium chloride as a reagent for selenium is justified. The proposed sample preparation procedure involves gas extraction of selenium as hydrogen selenide followed by its liquid-adsorption extraction from the gas phase to an aqueous reagent solution with the formation of a water-insoluble formazan. Formazan formed upon the absorption of hydrogen selenide is extracted with isoamyl alcohol. The concentration of selenium is determined from the absorbance of the formazan extract in the isoamyl alcohol. The procedure allows the determination of 10-120 μg/L selenium.