The prompt-neutron-induced fission spectra of 233U, 235U, 239Pu, and 240Pu were measured relative to the prompt-spontaneous-fission-neutron spectrum of 252Cf. Analysis of the measured values indicates that the incremental average-fission-neutron energies relative to a 252Cf fission-neutron spectrum of Maxwellian form with an average energy of 2.13 MeV are −123 ± 30 (233U), −157 ± 24 (235U), −76 ± 29 (239Pu), and −46 ± 29(240Pu) keV.