A fast coincidence technique was employed to measure the relative prompt-neutron yields per fission of U233, U235, Pu239, and Pu241 induced by thermal neutrons. The values obtained were ν23/ν25 = 1.02 + 0.01, ν49/ν25 = 1.23 ± 0.01, and ν41/ν25 = 1.295 ± 0.02, where ν23, ν25, ν49, and ν41 designate the prompt-neutron yields of U233, U235, Pu239, and Pu241, respectively.