A calculational study was made of the response of 237Np in a “threshold” fission detector to be used in a system for measuring the fissile content of spent fuel subassemblies by nondestructive assay. The results show that the contribution of the subthreshold fission component of this isotope to the total fission activity of the detector precludes the use of 237Np in this application, thus demonstrating the importance of including these usually ignored subthreshold fission cross sections in the data file.