THE defects in as-grown bulk YBa2Cu3O7 have been shown to act as weak flux pinning sites, leading to a reversible magnetization1, magnetization decay2 and non-zero resistivity3 in moderate magnetic fields at temperatures near the transition temperature, Tc. These effects limit the technological utility of the as-grown material3. This is not entirely surprising, as both single crystals and polycrystalline materials are grown under conditions expected to minimize the defect density, but the weak pinning in these materials has nevertheless raised the issue of a possible intrinsic limitation to the critical current density. Here we report the attainment of a critical current density, Jc, of ∼6 × 105 A cm-2 at 77 K and 9 kOe, in a single crystal of Yba2Cu3O7 irradiated with fast neutrons to introduce defects. (In both irradiated and unirradiated crystals, the critical current density was inferred by measuring the magnetic moment resulting from an induced persistent current.) This hundredfold enhancement of Jc relative to unirradiated crystals demonstrates that artificially induced defects can act as strong flux pinning sites, and analysis of the defects should increase our understanding of flux pinning in high-Tc superconductors.