The defect morphology of Se‐doped GaAs grown by CVD on undoped 100‐oriented GaAs substrates has been studied by transmission electron microscopy. Two types of samples are discussed: The first is nearly saturated,n≅NSe≅ 4 × 1018cm−3; the second is supersaturated,n≅ 1.5 × 1019cm−3,NSe≅ 4 × 1019cm−3. The nearly saturated samples exhibit an array of small features (probably Frank loops) near the original interface and have a density of misfit relieving dislocations≲103 cm−1. The supersaturated samples show a large number of Frank loops, whose density decreases monotonically with distance away from the original interface while their diameterincreasesin this direction. The loops seem to be invariably connected with one or more very small precipitate particles of Ga2Se3. Most of the Frank loops are of the intrinsic type; these are sometimes accompanied by closely neighboring loops of extrinsic nature. Pure edge misfit relieving dislocations with a density ≅ 104cm−1are observed at the interface, running in one of the⟨011⟩directions, but not in the orthogonal direction. A model is proposed to explain these results. Among the ingredients of this model are the notions (i) that misfit dislocations are asymmetrically introduced well away from the original interface and climb to that interface; (ii) that the solubility of Se in GaAs is a function of the imposed strain; and (iii) that the precipitation of Ga2Se3in GaAs is accompanied by the formation of both As vacancies and interstitial Ga in the matrix.