The twin textures of two differently synthesized types of YBa/sub 2/Cu/sub 3/O/sub 7- delta / single crystals, differing strongly in their critical current density (j/sub c/) levels, have been investigated by high-resolution transmission electron microscopy. One type of crystal which has higher j/sub c/ values and contains 1 at.% Sr shows twin lamellae with boundaries decorated by many small local strain fields. It is suggested that this twin boundary structure is due to Sr doping. The Sr clusters present in the lattice are assumed to enhance flux pinning, and thus to result in the higher j/sub c/ values observed.