Thin layers of Mo and Nb, 100-400 Å thick, were deposited onto clean (100) and (1̄1̄1̄)GaAs substrates under ultrahigh vacuum conditions in a molecular-beam epitaxy system, at slow rates and at relatively low temperatures. The microstructure of the films and the orientation relationship with the substrates were determined by in situ reflection high-energy electron diffraction, by transmission electron microscopy, and by grazing-incidence x-ray diffraction. In spite of the large lattice mismatch to GaAs (11% for Mo and 17% for Nb) and the low deposition temperatures(<400 °C), oriented deposits were obtained on both substrates for both metals. Although both metals are body-centered cubic with similar lattice parameters, and are both of similar chemical behavior, they grow differently on GaAs. Molybdenum grows epitaxially in the (111) orientation on both (100) and (1̄1̄1̄)GaAs substrates, whereas niobium grows with the (100) orientation on (100)GaAs, and with no simple orientation on (1̄1̄ 1̄)GaAs. In both cases, the orientational spread (deduced from the diffraction patterns) is smallest when the lattice planes parallel to the interface have the same symmetry in film and substrate.