[en] The fraction of delayed neutrons β (with T/sub 1/2/ greater than or equal to 0.025 s) in slow-neutron beams from a ²³⁸U pulsed spallation neutron source is 0.0053 for 300 MeV protons. This measurement appears to be the first one of this quantity. The result indicates that, for most classes of measurements, the delayed-neutron background in time-of-flight instruments will be unimportant, and places constraints on the physics description of spallation targets. The measurement was performed at the prototype pulsed spallation neutron source, ZING-P', at Argonne National laboratory. 4 figures