Microstructure and magnetic hysteresis have been compared for two samples of the YBa2Cu3O7−&dgr;(&dgr;∼0) high‐temperature superconductor, one unirradiated, and one irradiated with fast neutrons (E≳0.1 MeV) to a fluence of 3×1018n/cm2. Notable changes in the microstructure include strain‐induced contrast from regions 2–7 nm in size. An intrinsic critical current density (Jc) of 4.6×106A/cm2in zero field at 4 K has been determined from magnetic hysteresis measurements for the irradiated sample while 1.2×106A/cm2is noted for the unirradiated sample. We propose that the observed defect structure in the irradiated material is responsible for increased pinning and consequently higherJc’s.