Vero (African Green Monkey Kidney) cells infected with Chlamydia trachomatis (serovar L2) (CT-L2) for 48 hr and superinfected with herpes simplex virus type 2 (HSV-2) were fixed 24 hr later and examined with the transmission electron microscope (TEM) and scanning electron microscope (SEM). Ultrastructural examination of the co-infected cells showed that, although many CT-L2 inclusions were present, most were empty of reticulate bodies or elementary bodies. However, large numbers of viruses were observed in the co-infected Vero cells and, in some instances, were seen inside the CT-L2 inclusions. SEM studies of the co-infected cells revealed a swelling and rounding up of the Vero cells typical of a CT-L2 or HSV-2 infection. The outer surface of the co-infected cells were covered with many long microvilli typical of HSV-2 infection with only a few surface protruberances similar to those found in CT-L2 infected cells.