Three types of aromatic polyamide fibers (Kevlar, Kevlar-49, and PRD-49) were studied by transmission electron microscopy, secondary electron imaging, and electron diffraction techniques. Ultramicrotomy, ion-thinning, and brittle fracture techniques were used to prepare specimens which revealed both longitudinal and transverse fiber structures. Aromatic polyamide fibers exhibit both a “skin” and a “core” structure. The core morphology is shown to be consistent with a layered structure where individual layers are stacked perpendicular to the fiber axis and are composed of rodlike crystallites whose c axes are parallel to the fiber axis. Electron diffraction results indicate that hydrogen bonds are located nearly in the radial direction of the fiber. The crystallite layer thickness appears to depend upon the most probable molecular weight of the fiber. Longer crystallites which pass through two or more layers are present, and may control the strength of the fiber in the axial direction.