The determination of selenium by atomic fluorescence spectroscopy has been reported recentlyinteraliaby Dagnall, Thompson and West1,2, and by Marshall and West3. These workers obtained detection limits of 0.15 p.p.m. using an instrument with a slit width ofca. 2mm, and 10 p.p.m. using an instrument with a slit width of 0.lmm respectively, using air-propane flames. The apparatus used in this investigation consisted of a Southern Instruments A 1740 grating flame photometer fitted with Hamamatsu U.V. sensitive photomultipliers and a grating blazed at 300 nm. The monochromator provided with this instrument provides facilities for integration and automatic flame background correction, but has a fixed slit width of only 0.lmm. Using this instrument in conjunction with a selenium electrodeless discharge tube source operating at 2450 MHz and an air-acetylene flame, a detection limit of 1 p.p.m. was obtained at 196.lnm for aqueous solution when an integration period of 20 secs. was used. The A 1740 monochromator was then reglaced by an A 1750 nonochromator which has a slit width variable between 0.025 and l.0mm. When a Imn slit was used in conjunction with flame background correction and a 20 sec. integration period, a detection linit of 0.2 p.p.m. was obtained. In this instance, the detection limit woras taken as the concentration for which the signal is twice the standard deviation.