[en] A delayed-neutron assay method is described which is capable of accurately determining the ²³⁵U and ²³⁸U contents of a wide range of materials. The method consists of irradiation in the Coupled Fast Reactivity Measurement Facility (CFRMF) at the Idaho National Engineering Laboratory (INEL), one position in the thermal driver area for ²³⁵U assay, and a fast neutron flux position for ²³⁸U. After irradiation, delayed neutron counting is used to measure the uranium content of the sample. By utilizing fast and thermal neutron irradiation and counting cycles, the samples ²³⁸U and ²³⁵U contents are determined. Minimal sample preparation and handling are required to obtain accurate results. The method has been used successfully on a wide range of samples, including environmental (soil) samples, irradiated and unirradiated fuel, and debris from the Three Mile Island Unit II Reactor core. Measurement precision of up to 0.5% can be achieved; sensitivity for low level samples is 9 counts min^-1 nanogram^-1 for ²³⁵U and 0.02 counts min^-1 nanogram^-1 for ²³⁸U. The fissile and/or fertile contents of samples containing ²³³U, ²³⁹Pu, ²⁴⁰Pu, ²³⁷Np, or ²³²Th could be determined by the same method. Data from representative samples are presented to illustrate the usefulness of these techniques