[en] The energy dependence of the average number of prompt fission neutrons emitted per fission, anti ν/sub p/(E), has been measured for ²³³U relative to anti ν/sub p/ for ²⁵²Cf over the neutron energy ranges 500 eV to 10 MeV and below 0.3 eV. A large Gd-loaded liquid scintillator was used to detect neutrons and the samples of ²³³U and ²⁵²Cf were contained in fission chambers. The present results for anti ν/sub p/(E) for ²³³U are in accord with the experimental results of Boldeman and the evaluated results of Lemmel in the thermal energy range, but in the neutron energy region between 100 keV and 1 MeV the present data are 1% or more larger than other experimental values