Beneath a 2 ´ 10⁶ km² area of the hyperarid Western Desert of Egypt and adjacent portions of eastern Libya, northeastern Chad, and northwestern Sudan lies an immense reservoir of fresh water in the Nubian Aquifer system.  This is a fossil groundwater reservoir with recharge that is minor compared to natural discharge and pumping for agricultural and industrial use.  Its water volume (~5 ´ 10⁴ km³) is equivalent to about 500 years of Nile River discharge.  Although there have been a large number of radiocarbon analyses of Nubian aquifer water, indicating that some samples have ages (mean residence times) ≥ 5´10⁴ years, and hydrologic models indicate ages up to 10⁶ a, there has not yet been a definitive measurement of the age of the Nubian aquifer water and its variation along flowpath.   An outstanding question is whether recharge has been restricted mainly to the highlands in the southwest, or has been distributed regionally, during humid climate periods during the Quaternary; each of these recharge modes results in a different distribution of natural radioactive tracers within the aquifer.  We address this question using measurements of ⁸¹Kr, a new tool for groundwater dating.  ⁸¹Kr has been recognized as an ideal tracer for groundwater dating in the 50,000–1,000,000-year range, but its low abundance (~10³ atoms/L H₂O) has made routine measurement nearly intractable.  We have applied a new analytical method, atom-trap trace analysis (ATTA) (Chen et al., 1999, Science), to measure the ⁸¹Kr abundance in samples of Nubian aquifer groundwater.  Six groundwater samples were taken from wells screened in the deeper portion of the aquifer (600–1200 m) in major oasis areas of the Western Desert (Baris, Kharga, Dakhla, Farafra, Bahariya).  Dissolved gas was bulk-stripped from water in the field.  The Kr was separated from the bulk gas and analyzed by low-level counting (LLC) for ⁸⁵Kr at Bern.  It was then spiked with ⁸⁵Kr, analyzed again by LLC, and shipped to Argonne for analysis of ⁸¹Kr/⁸⁵Kr by ATTA.  ⁸¹Kr ages of the water samples, obtained by comparing their ⁸¹Kr/Kr ratios with that of modern air, range from ~200 to ~1,000 ka.  Ages increase with distance along the flowpath and correlate well with ³⁶Cl/Cl ratios.