| Declining population sizes of koalas (Phascolarctos cinereus) in SE Queensland, Australia can partially be attributed to chlamydiosis, the most documented and serious disease of koalas, characterized by ocular, urinary, and reproductive lesions. Although the disease is not necessarily fatal, chlamydiosis results in hundreds of koalas being presented to koala hospitals in Queensland each year; these animals are either treated or euthanased. Studies related to the incidence and pathology of Chlamydia infection and its effect on male reproduction are extremely limited, so that the impact of this disease on wild koala populations has most likely been underestimated. This presentation will explore the epidemiology of Chlamydia pecorum infection in the male urogenital tract from wild (hospitalized and free-ranging) koalas in SE QLD, investigate whether Chlamydia-induced pathology can either be a direct negative effect on the sperm cell or an indirect effect associated with infection leading to inflammatory obstruction of the seminiferous tubules and/or epithelial damage that results in impaired spermatogenesis and sperm transport. Furthermore, this study presents the first empirical evidence that Chlamydia is indeed a venereal transmitted disease in the koala, as demonstrated by successfully inoculating a cell line in vitro with naturally-infected koala semen.