Abstract A number of authors have discussed the pinning of magnetic fluxoids in high temperature superconductors in terms of a cooperative process involving large numbers of defects. In particular, in a recent study of magnetic flux creep in high quality single crystals of YBa 2Cu 3O 7−δ, we showed that an appropriate collective pinning model accurately described the creep in that part of the H-T phase diagram where the creep is controlled by single fluxoid motion. Here we present data on similar crystals given an additional heat treatment at temperatures above 800°C. These crystals exhibit large critical currents that exceed the limit associated with collective pinning, suggesting the influence of strong pinning defects. An analysis of the pinning potential U(j) at large j provides further evidence for strong pinning. We suggest that the heat treatment introduces precipitates of nearly optimum size for pinning, leading to a combination of weak and strong pinning defects.