Background: Seasonal anestrus, which is controlled by the photoperiod in ewes, limits the ability to the fertility of ewe every time. Therefore, using hormones by vaginal devices is the most effective way of increasing fertility in order to form oestrus during the anestrus period and obtain a high rate of pregnancy. But, vaginal devices may cause a change in the profile and number of bacteria in the uterus and vagina, and thus create an environment suitable for infection. For this reason, identification of bacteria that are the source of infection can be achieved by performing antibiotic susceptibility tests and interpreting them correctly. The purpose of this study was to characterize the effect of intravaginal device-type and treatment-duration on vaginal bacteria count and type and determine the antibiotic susceptibility of vaginal bacterias in ewes after a program of oestrus synchronization.