An occurrence of a workable deposit of tremolite asbestos has originated by the alteration of a body of ultrabasic rock intrusive into the Archean crystalline rocks of the Singhbhum District, Bihar, India. Although there is microscopic evidence of the formation of antigorite or chrysotile from olivine, and tremolite from diallage, lime metasomatism has converted the chrysotile into logs of tremolite asbestos. Chrysotile asbestos also occurs, but it is subordinate to tremolite. Serpentinization was caused by hydrothermal solutions derived from some unknown deep source since younger acid intrusives are lacking in the area. Olivine grains have been affected uniformly, and serpentinization is not confined to their cores or margins, nor to any pre-existing fissures. The nonfibrous serpentine was transformed into fibrous asbestos by stress acting along certain zones. The ultra basic bodies are not inclusions in the granite gneiss but represent younger intrusions.