An electron microscopic study was done on a glomus jugulare tumor from a 69-year-old male. With the electron microscope, it was found that most of the tumor cells and their processes seemed to be in close contact with capillaries, whose endothelial cells were fenestrated by pores. The most significant structure in the glomus tumor cell is a large number of membrane-bounded electron dense granules, morphologically similar to the catecholamine-containing granules. The functional significance of the occurrence of such granules is discussed.