The lattice constants and the atomic position parameter,z, of a high purity, low strain, single‐crystal of arsenic have been determined. Low extinction reflections of filtered MoKα and AgKα radiation were used for the determination ofz. The Bond precision technique with MoKα radiation was used for the determination of the unit‐cell dimensionsaandc. Average values foraandcin Å, and forzare: 4.2°Kz= 0.22764a= 3.7597c= 10.4412; 78°Kz= 0.22754a= 3.7595c= 10.4573; 299±3°Kz= 0.22707a= 3.7598c= 10.5475. The estimated standard deviation ofzis ±0.00004 at 4.2°K, ±0.00002 at 78°K and ±0.00005 at 299°K. The precision of a is estimated to be ±0–0001 Å and that forcto be ±0.0002 Å. The results for As are compared with those