Rod-like projections on the surface of Chlamydia trachomatis have been studied by a combination of computer image analysis and electron microscopy. The rods, c. 60-80 Angstrom in diameter and c. 500 Angstrom in length, were found on the surface of prokaryocells of C. trachomatis inserted in the cytoplasmic membrane through a ring-like structure in the outer membrane. The rod-like structures were found at all stages of the life cycle, even in very small elementary bodies (EBs) of C. trachomatis and in vesicles, 0.2 mu m. Computer image analysis of isolated rods indicated that they comprise helically arranged subunits with a periodicity of c. 50 Angstrom. From their localisation and distribution, these structures may be related to the proliferation, or to the infectivity, of chlamydiae.