The 237Np(n,f) cross section has been measured relative to the 235U(n,f) cross section from 20 eV to 7 MeV, including the range where no data previously existed (4 to 24 keV). Energies of the source neutrons from the Physics 8 underground nuclear explosion were determined by measuring the time-of-flight from the source to the neptunium target foil. The current signals produced in solid-state detectors by fission fragments were photographically recorded and converted to cross sections using the neutron flux measured with 235U(n,f)and 6Li(w,α)T reactions. Subthreshold resonances were observed in the 237Np(n,f) cross section. The maximum value of the cross section observed just above threshold is 1.9 b at 2.26 MeV. A comparison of results with some previously reported values is presented.Present address: University of Wyoming, Laramie, Wyoming 82070.