The lifetime of the 279 keV excited state of e°aTl was determined to be 0.28±0.04 nsec by the method of nuclear resonance fluorescence, using the 212 keV beta radiation for compensation of the recoil energy loss. Using gaseous and liquid sources and a thallium scatterer, the cross sections for resonance fluorescence and Rayleigh scattering processes were determined. The agreement between theory and experiment on Rayleigh scattering cross section obtained from the present measurement lends support to this value of lifetime. E NUCLEAR REACTION 2°aTl(y, 7); measured ~(0), resonance fluorescence. ] 2°aTl level deduced T,.