Optically induced dark‐line defects (DLD’s) in GaAs‐Ga1−xAlxAs double‐heterostructure (DH) wafers have been observed by x‐ray topography for the first time using the x‐ray anomalous transmission effect. This topography, which is sensitive to defects in the thin epitaxial layers, has been successfully applied to the observation. As a result, it has been observed that DLD’s oriented along ⟨110⟩ directions are composed of dislocations with Burgers vectors of (1/2)a⟨110⟩ parallel to the growth surface. It is also shown that the DLD’s originate from both substrate dislocations and crystal edges.