The total neutron cross sections of representative, single‐crystal samples of bismuth and quartz have been measured at room and liquid nitrogen temperatures over the neutron energy range of 0.0007 to 2.0 eV to obtain data for the design of single‐crystal filters for attenuating fast neutrons and gamma rays in reactor beams. The epithermal neutron and gamma‐ray transmissions of these crystals, calculated from cross sections given in various compilations, are also presented graphically. From this information, filters composed of one or both crystals may be constructed to optimize beam characteristics for low energy neutron experiments. The design and performance of the 32‐cm bismuth, single‐crystal filter at the Danish DR‐3 reactor are described. The average thermal transmission of a typical reactor beam through this filter is 16%. In the wavelength region from 5 to 15 Å, the transmission is 72%. Neutron intensity in the resonant energy region is attenuated by a factor of 5000, and gamma rays of all energies by more than 5×105. The filter improved the Cd ratio of the reactor beam by a factor of 800.