The GaAs (1̄ 1̄ 2̄)B surface was prepared by molecular beam epitaxy and investigated by in situ scanning tunneling microscopy with respect to structure and morphology. A remarkable surface corrugation with (0 1̄ 1̄) and (1̄ 0 1̄) facets was observed at Ga-rich condition. The period of the corrugation is about 12 nm. The Ga-rich structure changed into an As-rich structure, when the sample temperature was decreased to below about 520 °C under As 2 flux. The As-rich surface is also not flat but undulated on a sub-μm scale. On an atomic scale it is rather rough being composed of fragments of 1D chains along [1 1̄ 0]. There is a twofold periodicity along [1 1̄ 0], but little periodicity along [1 1 1̄].