Single crystals of the compound silicon hexaboride have been prepared. Laue, rotation, and Weissenberg photographs indicate an orthorhombic cell with lattice parameters , , and . The pycnometer density is 2.43 g/cm3. The average electrical resistivity of single crystals, based on a probe technique, is 0.2 ohm‐cm. The melting point is 1950°C. has a Knoop hardness, with a 100‐g load, of 1910 kg/mm2.