Biehlite, a new diantimony molybdenum hexaoxide, ideally (Sb,As)2MoO6, is monoclinic, space group C2lc, with a = 18.076(5) Å, b = 5.920(5) Å, c = 5.083(5) Å, β = 96.97° (1), Z = 4, cell volume 539.91 Å3, from single crystal studies. The strongest reflections in the X-ray powder diffraction data are (dmeas., I, hkl) 5.622 (65) (110), 3.376 (39) (311), 3.104 (61) (311), 2.990 (100) (600), 2.960 (100) (020), 2.104 (42) (620). Biehlite was found in the Tsumeb mine, Tsumeb, Namibia. It forms long white flexible fibreous crystals which are only some microns in diameter. The fibres build up irregular aggregates and felty masses in association with anglesite and wulfenite. Electron microprobe analyses give Sb2O3 60.99, As2O3 4.95, MoO3 33.76, total 99.70 wt.%, leading to an empirical formula of (Sb3+1.79, As3+0.21)Mo6+1.0006.