Examination of asbestos fibers by electron microscopical techniques enables the observer to distinguish among the fiber types by morphological and structural characteristics. Chrysotile asbestos fibers are composed of bundles of fibrils. Fibers are often curvilinear with splayed ends. Individual fibrils consist of a central capillary defined by an electron dense crystalline wall. With increasing time of electron bombardment, the capillary wall decreases in thickness, deforms, and is encapsulated in an electron translucent material. The change in electron opacity is considered to be a product of structural disruption brought about by dehydroxylation due to electron radiation. A well recognized sequential deformation pattern may be used for identification purposes.Amphibole fibers tend to be straight, splintery, and electron-opaque, although curved fibers are occasionally observed. Diffraction contrast figures are visible as dark bands moving parallel and at right angles to the fiber axis. Crocidolite forms the shortest and thinnest fibers, followed in size by amosite and anthophyllite. Size distribution characteristics of the amphibole fiber types are different. The selected area electron diffraction pattern for chrysotile is unique. Reflections range in forms from streaked to arcuate. Reflection intensity and shape are related to the degree of openness of the fiber bundle and the extent of physical degradation of the fiber. The amphibole asbestos fibers possess diffraction patterns having similar characteristics prohibiting individual identification. Microchemical analysis is required for identification in such cases.A discussion of the industrial hygiene threshold limit values for ampliphibole asbestos fibers is presented. The discussion is based on their differing size distribution characteristics.