We have used electron microscopy to follow the course of Chlamydia trachomatis infection in the mouse cervix. Although numerous elementary bodies (EBs) were observed on the surface of epithelial cells, evidence of coated pits or entry of EBs into epithelial cells were rarely observed. After 2 days postinoculation, inclusions contained numerous reticulate bodies (RBs) and a few intermediate forms (IBs). At 4 days postinoculation, microvilli were no longer present on infected cells and inclusions had often ruptured and released chlamydiae into the cytoplasm of the cells. Aberrant and miniature RBs, similar to those which have been described in in vitro models for persistence, were observed. Unlike the case in vitro where inclusion cause rupture of the cell, infection in vivo may result in rupture of inclusions within the cytoplasm of infected cells. Our observations also suggest that persistent chlamydia can form in some of the cells of the cervical epithelium that are infected by the initial inoculation.