The determination of selenium by hydride generation atomic absorption spectrophotometry using zinc is widely used. Very little is mentioned, however, on possible interferences. In an attempt to use this technique for the determination of selenium in natural waters, interferences were found. The most serious interference occurred when arsenic was present. As little as 5 μg of arsenic/l suppressed the formation of selenium hydride. Because arsenic may be present with selenium in natural waters, the above method was abandoned in favor of the following atomic absorption spectrophotometric method for the determination of both inorganic and organically bound selenium. Organic selenium compounds, if present, are first decomposed by digestion with potassium permanganate in hot acid solution. The solution is then made basic and the solution evaporated to dryness in the presence of calcium chloride which prevents the loss of selenate during the evaporation. Hydrochloric acid is added to the residue to reduce the selenate to selenite. Quantitative reduction is obtained without loss of selenium by controlling the temperature, time, and acid concentration. It should be emphasized that the selenium must be in the selenite form prior to its final reduction. The selenium is finally liberated from 6M hydrochloric acid solution with stannous chloride and the selenium hydride vapor subsequently removed from solution by aeration and swept by a flow of nitrogen into a hydrogen flame where it is determined by atomic absorption at 196.0 nm. The method may be used to analyze water samples containing at least 2 μg/l of selenium.