Selective‐area epitaxial growth of gallium arsenide on n‐Si(100) substrates is reported, where the oxide (SiOx) mask consists of 1–2 monolayer‐thick features patterned onto a silicon substrate using a scanning tunneling microscope (STM) operating in air. The technique for generating the STM patterns on hydrogen‐passivated silicon was reported recently [J. A. Dagata, J. Schneir, H. H. Harary, C. J. Evans, M. T. Postek, and J. Bennett, Appl. Phys. Lett. 56, 2001 (1990)]. The GaAs epilayer was grown by migration‐enhanced epitaxy at 580 °C and its morphology was investigated by scanning electron microscopy. The chemical selectivity of the STM‐patterned regions was verified by imaging time‐of‐flight secondary‐ion mass spectrometry. The implications of these results for the development of a unique, STM‐based nanostructure fabrication technology are discussed.