Three types of commercial rayon (Fortisan, high-tenacity tire yarn, and modifiertype, high wet modulus yarn) were grafted with styrene, using the direct radiation method with gamma-rays from Co-60. The grafted filaments were examined in the electron microscope to investigate their morphology and fine structure, the location and distribution of polystyrene in the fiber, and whether the polystyrene was truly grafted.In all three grafted fibers, the polystyrene was distributed throughout the cross section of the filament and also appeared on the surface. As a result, the filaments expanded isometrically during grafting, but retained their original shape. The polystyrene grafted to the surface of the filament followed the original surface striations of the rayon and appeared as protruding ridges and valleys, rather than as uniform surface layers. The polystyrene inside the filament could not be removed hy exhaustive extraction of the intact yarn with benzene, but some of it could be removed by extracting the ultra-thin cross sections (ca. 500 & A r i n g ; thick) used for electron microscopic observations. The main portion of these ultri-thin sections was insoluble in solvents for both polystyrene and cellulose and is considered to be a true cellulose-styrene graft.The changes induced in the fine structure of the cross sections by alternate extraction with solvents for polystyrene and cellulose were followed, using the electron microscope. From a macroscopic viewpoint, it was concluded that the styrene grafted rayons are homogeneous. At a molecular level, however, they consist of a gross heterogeneous mixture of polystyrene homopolymer, cellulose, and true cellulose-styrene graft copolymer.