The microstructure of Czochralski silicon annealed at high temperatures has been investigated using high resolution transmission electron microscopy. After prolonged heat treatments at temperatures close to 1200°C, two types of microdefects are observed: (a) small polyhedral oxide precipitates, with typical diameters of about 15 nm and facets along crystalline planes of the silicon matrix, and (b) planar faults along {111} planes which have been directly identified as extrinsic stacking faults bounded by Frank-type dislocations.