The ability of the housefly to carry viable Chlamydia trachomatis and to transmit a chlamydial ocular infection was studied under laboratory conditions. After feeding flies (Musca domestica) on suspensions of egg yolk sac infected with C. trachomatis serotypes A or B (responsible for hyperendemic trachoma) the agents were reisolated from flies' intestines for up to 6 hours and from their legs and/or proboscises for up to 2 hours. It was found that the viability of chlamydiae is dependent on the protective effect of yolk concentration in the original inoculum. Results of experiments with guinea-pig inclusion conjunctivitis as an animal model show that under laboratory conditions flies can readily transmit this chlamydial ocular infection from one animal to another. These results suggest that under field conditions flies can play an important role in the transmission of trachoma, particularly in areas with favourable conditions such as a large reservoir of infection among children with severe trachoma, copious eye discharge caused by trachoma and associated bacterial infections, a large fly population, and close proximity of children in large family groups.