We report on the simultaneous, molecular beam epitaxy growth of GaAs on GaAs(001) and GaAs(111)Bsubstrates at low temperatures. The crystallinity of the low‐temperature GaAs layers was assessed using a double‐crystal x‐ray diffractometer and a wide‐angle diffractometer with a rotating specimen stage. Layers were grown at 200 and 250 °C to a thickness of 3 &mgr;m on both (001) and (111) orientated substrates and an additional 3 &mgr;m layer was grown on GaAs(111)Bat 300 °C. Double‐crystal diffractometry confirmed the presence of a single crystalline layer, with a growth‐temperature‐dependent excess As concentration, on the (001) substrates. On the (111) substrates, only a polycrystalline layer was observed. A possible explanation for these observations based on growth surface roughening is presented.