A dry etching process is described for the fabrication of through-wafer via holes in GaAs-based monolithic microwave integrated circuits by magnetron ion etching using SiCl4. The process has been used to etch via holes through 80 mu m GaAs substrates with good anisotropy and etch rates exceeding 1 mu m/min for via holes as small as 18x20 mu m(2). Etch anisotropy appears to be due to an etch-inhibiting layer, which forms on the sidewalls but can be removed readily using a brief SF6/O-2 etch. The process also exhibits good selectivity to Au and Ni frontside metal.