The fission yields for Ba139 and Ba140 for thermal neutron fission of U235 and fast fission of U238 have been measured using naturally occurring uranium. This was accomplished by comparing the barium fission product activities with the U239 formed in radiative capture. With these data the fission yields were calculated from known values of the cross-sections for thermal fission and capture in uranium and were found to be 6.1% and 5.6% for Ba139 and Ba140, respectively, for thermal fission and 5.1% and 4.2% for Ba139 and Ba140, respectively, for fast fission. The contribution of resonance fission to the thermal neutron effect has been shown to be less than 5% in the irradiation arrangement used.