The crystal structure of BiSCl has been redetermined with threedimensional intensities, measured on a computer-controlled Philips PW1100 single-crystal diffractometer (628 independent reflexions). The structure does not deviate in principle from that proposed by Dönges (1950), but it shows interesting details. The cell constants, obtained by least-squares calculations from direct 3-value measurements on the diffractometer, are: a = 7.7508(16), b = 9.9920(5), c = 3.9955(5) Ä, Ζ = 4; the space group is Pnam. The positional and vibrational parameters, with anisotropic temperature factors, were refined by full-matrix, least-squares calculations to a final R = 0.036.Each Bi atom is seven-coordinated by three S atoms at distances 2.605 -2.711 Ä and four chlorine atoms at distances 2.927-3.367 Ä. The four CI atoms form a square parallel to c, while the three S atoms form an isosceles triangle perpendicular to (001), with its base parallel to c. The four CI and two of the S atoms are at the vertices of a triangular prism parallel to c and of height equal to c. The third S atom is at the apex of a pyramid whose base is one of the S 2 C1 2 faces of the triangular prism. The composite coordination polyhedron (prism-pyramid) is linked to a symmetry-equivalent polyhedron by a common S -S edge of the corresponding pyramids. This characteristic double-polyhedron system is repeated along the c axis, thus forming an infinite double chain of composition {Bi 2 S 4 Cl 8 }". The chain is further linked to each of four similar chains by a common (CI -Cl)" row parallel to c. This system of chains explains very well the needle-like growth of the crystals along c.