The prompt neutron period of a bare sphere of U235 has been measured at two reactivities between delayed and prompt critical. The x-ray burst from a betatron was used to establish the initial neutron population in the critical assembly, and the neutron intensity was observed by photographing oscilloscope traces of pulses from a spiral fission chamber. The values obtained for α, the reciprocal of the prompt neutron period, are —0.52 ± 0.03 × 106 sec-1 at 54 cents reactivity, and —0.26 ± 0.03 × 106 sec-1 at 76 cents.