Chlamydiae, which are obligate intracellular bacterial pathogens, have been radiated from single-celled eukaryotes into multi-celled hosts during their evolution. Chlamydia trachomatis one of the important species in this group, is classified into three biovars as a result of their evolution. Two of those biovars, Trachoma and LGV, are pathogens only in humans. Initially, the presence of a high specificity between the host and chlamydiae has been recognized and this relation has been considered as an adaptation mechanism. However, some studies have indicated that chlamydiae can also grow in laboratory animals, yolk sacs of embryonated eggs and in vitro cell cultures. The aim of this study was to investigate if C.trachomatia human specific biovars are possible infectious agents in the aborted bovine fetuses. Ninety aborted bovine fetuses were included in the study, and the bacteria which could be the causative agents for abortion were searched by conventional microbiological methods. Twenty-three (25.6%) abortion materials which have yielded negative results with these methods for the presence of bacterial agents other than chlamydiae, were further evaluated in terms of the presence of C.trachomatis. For this purpose the samples were inoculated into the yolk sac of embryonated eggs and the slides prepared from the yolk sac membranes of embryons died after 24 hours of inoculation, were examined for the presence of inclusion bodies by staining with Giemsa method. The presence of C.trachomatis specific antigens and glycogen inclusions in those 23 samples were also investigated by immunohistochemical and Lugol's iodine staining methods, in the fetal tissue samples which were embedded in paraffin. Immunohistochemical method was performed with immunoperoxidase staining by the use of specific antibodies against C.trachomatis major outer membrane proteins. As a result, 5 (21.7%) of the 23 samples were found positive for C.trachomatis with three of the methods (Giemsa, immunoperoxidase and lugol stainings). Although the data of our study have supported that chlamydiae can adapt to new host species other than humans, further advanced studies are needed on this subject. Our results have also emphasized that novel routes of transmission should be considered for C.trachomatis infections.