The defect structure of a 350-nm-thick epitaxial Ba0.3Sr0.7TiO3 (BSTO) film grown on (001) LaAlO3 has been investigated using conventional and high-resolution transmission electron microscopy. The dominant defects in the film are edge-type threading dislocations (TDs) with Burgers vectors b=<100> and <110>. Pure-screw TDs and partial TDs of mixed character were also observed. A rapid reduction of defect density occurred after the growth of the first 100 nm BSTO adjacent to the interface. In the top layer of the film, all TDs with b=<100> are perfect while those with b=<110> are usually dissociated into two partials with a small separation (a few nanometers). However, in the near-interface layer of the film, many TDs with b=<100> are split into two or three partials. A high density of extended stacking faults with displacement vectors of 1 2 <110> type were observed. The stacking faults are associated with dissociated dislocations and partial half loops. The mechanisms for the generation, dissociation and evolution of the TDs are discussed. © 2003 American Institute of Physics.