Striking changes in membrane systems occur in the vicinity of replication complexes and mitochondria in rubella virus (RV) infected Vero cells. An electron-dense zone about 22-25 nm in thickness fuses membranes in 3 configurations: between the outer membrane of a mitochondrion and one membrane of the rough endoplasmic reticulum (RER), between the outer membranes of two adjacent mitochondria, and between two apposing membranes of the RER. These junctions were called confronting membranes type 1 (CM-1), confronting membranes type 2 (CM-2) and confronting cisternae (CC), respectively. CM-1, CM-2 and CC were not observed in mock infected or Semliki Forest virus (SFV) infected cells. However, mitochondria were found to cluster around replication complexes in both SFV and RV infected cells. This suggests that replication complexes are sites of high energy requirement because of their key role in virus replication. The electron-dense zones appear to create almost continuous links between RV replication complexes and mitochondria.