Thin films of the high-temperature superconductor YBa2Cu3O(7-δ) exhibit both a large critical current (the superconducting current density generally lies between 10 and 10 A m-2 at 4.2 K in zero magnetic field) and a decrease in such currents with magnetic field that point to the importance of strong vortex pinning along extended defects. But it has hitherto been unclear which types of defect - dislocations, grain boundaries, surface corrugations and anti-phase boundaries - are responsible. Here we make use of a sequential etching technique to address this question. We find that both edge and screw dislocations, which can be mapped quantitatively by this technique, are the linear defects that provide the strong pinning centres responsible for the high critical currents observed in these thin films. Moreover, we find that the superconducting current density is essentially independent of the density of linear defects at low magnetic fields. These natural linear defects, in contrast to artificially generated columnar defects, exhibit self-organized short-range order, suggesting that YBa2Cu3O(7-δ) thin films offer an attractive system for investigating the properties of vortex matter in a superconductor with a tailored defect structure.@en