Summary: Jamesonite from Cornwall (type locality) and several localities in Bolivia gives identical X-ray powder photographs. Rotation and equi-inclination Weissenberg photographs on a needle from the Itos mine, near San Felipe de Oruro, Bolivia, give monoclinic symmetry. Spacegroup, C2h5=P21∕a. Cell edges, a 15·68±0·05, b 19·01±0·05, c (needle axis) 4·03±0·01 Å.; β 91° 48′±30′. Twin-plane (100). Cell content, Pb8Fe2Sb12S28 = 2[4PbS.FeS.3Sb2S3]. Sp. gr. 5·63 (highest of 7 measurements on different samples), 5·67 (calculated). Polished sections are white, strongly anisotropic; they show good basal and longitudinal cleavage. The powder pattern agrees with that obtained by Hiller (1938) on jamesonite from fourteen localities. Jamesonite may be distinguished from boulangerite by a microchemical test for iron, which gives a strong positive reaction with jamesonite, and by a positive etchtest with KOH on polished sections.