A series of experiments was performed to measure the dependence on the incident neutron energy of the average number of prompt neutrons emitted per fission from 233U, 235U, and 239Pu relative to the average number of prompt neutrons emitted in spontaneous fission of 252Cf. The incident neutron energy range was 0.0005 to 10 MeV. A white neutron source was generated by the Oak Ridge Electron Linear Accelerator, and the energies of the neutrons incident on the fissile samples were determined by time-of-flight techniques. In each experiment the samples, including the 252Cf standard, were contained in a fission chamber surrounded by a large volume (0.91 m3) of liquid scintillator loaded with gadolinium. The fission chamber detected the fission events, and the scintillator detected the accompanying prompt neutrons. The resulting data were analyzed to yield: p(E) = p(E) (fissile)/p(252Cf). For 235U and 239Pu our results overlap, within the experimental uncertainty, the results of the evaluation of Manero and Konshin (1972), and in the case of 235U our data show the same general structure apparent in the evaluation up to 0.5 MeV. Our p(E) for 233U does not show the structure near 0.2 MeV obtained by Manero and Konshin.