AbstractA new method has been proposed for the selection of a single resonance structure or ladder used for the calculation of the cross sections in the unresolved resonance region. This method is especially useful for fissile nuclides, where level spacings are quite narrow, and is capable of taking the rather complicated energy-variation of alpha values into consideration. By use of the present method, studies have been made of the cross sections of235U,238U,239Pu, and240Pu. For the construction of the cross sections of these nuclides, new evaluations have also been made of the average resonance parameters, which were used in generation of the ladders. The calculated results are shown to be a very good representation of the low-resolution experimental data.