The delayed-neutron fractions for thermal fission of 235U, 239Pu, and 233U were measured and found to be The technique used to perform the measurements was intrinsically simple and involved making only relative measurements. The basic approach was to compare the neutron production rate of a thin fissile sample in a thermal-neutron beam with the delayed-neutron production after an abrupt termination of the beam. The work was greatly enhanced through the use of a modified long counter that was developed at the Los Alamos Scientific Laboratory. The results of this work are in essential agreement with the presently accepted values, which were reported in 1957 and were measured by using a different technique than that used here.