New ultrastructural findings within the nucleus of herpes simplex virus-infected cells are illustrated. The occurrence of bundles of tightly packed filaments during the late stages of the infectious process is described. These bundles were found in different areas of the nucleus and were not associated with any nuclear organelles. Employing different staining techniques and high-resolution autoradiography, they could be identified as DNA-containing nucleoproteins. These filaments may be interpreted as a special form of viral DNA accumulation within the nucleus. Changes of these structures toward a more recticular arrangement were observed as soon as the nucleoplasm disintegrated.