The crystal structure of gustavite Pb1.06Ag0.95Bi2.80Sb0.20S5.99 from Rotgu¨lden, Lungau, Salzburg Land, Austria, was refined to R1 ¼ 2.76 % using single-crystal X-ray diffraction data. Refined unit-cell parameters are a 7.0567(14), b 19.6905(39), c 8.2219(16) A ° , b 106.961(3) , unit-cell volume 1092.7(2) A ° 3 and space group is P21/c. The structure contains a bicapped trigonal prismatic Pb site, a [001] string of Ag and Bi1 octahedral sites and a similar string of Bi2 and Bi3 coordination octahedra. Sb is concentrated in the Bi3 site whereas Bi1 and Ag contain a fraction (0.1–0.14 atom) of the opposite species; the latter is explained by a presence of twin lamellae or antiphase domains in the crystal. In accordance with its frequently observed twinning, gustavite is an OD (order-disorder) structure that can be described by zig-zag unit layers with layer symmetry P(2/n) 21/c 2/m and the interlayer
symmetry elements {(22/n1,1/2) 21/n1/2,2 21/2/ a2}. An alternative OD description uses planar unit layers with the layer symmetry P(2/
b) 21/c 21/m and the interlayer elements 22/n1,–1/2 21/n1/2,2 21/2/a2. The former description adheres closely to the details of coordination of bismuth observed in the structure whereas the latter description is based on a more abstract structure model.