Structural changes in the cells of a continuous cell line of hamster origin infected with A/NWS (HoNi) influenza virus were studied by electron microscopy. From the sixth hour on, inelusions consisting of fibrillar structures 35–45 Å in diameter were observed in the cell nucleoli. These inclusions gradually dispersed over the nuclear area and then penetrated into the cytoplasm through the pores of the nuclear membrane. In the cytoplasm they presented new morphological aspects: the fibrils were more regularly oriented, and the inclusions were surrounded by ribosom