Effects of three extrinsic incubation (EI) temperatures on vector competence of Ornithodoros savignyi to Borrelia sp. isolated from a natural population of the tick in Egypt were investigated.  The EI temperature influenced the efficiency of Borrelia sp. transmission by the female O. savignyi to hamsters.  An EI temperature for 27^oC was optimum for successful transmission of Borrelia sp. where the highest infection rates (IRs) in hamsters occurred.  Generally, the IRs in hamsters decreased by lowering the EI temperature to 17^oC or rising it to 37^oC in each of the corresponding experimental cases (number and postinfection period of the tick feeding per hamster) tested.             Ornithodoros savignyi kept at 27^oC and 37^oC maintained borrelial infection transstadially from nymph 1 to 7 and adults.  The transstadially infected stages transmitted the spirochetes to hamsters with higher IRs at 27^oC than 37^oC.  Transovarial transmission of Borrelia sp. occurred during the first and second (at higher rates) gonadotrophic cycle in the infected female ticks kept at 27^oC and 37^oC.  However, no molting or oviposition occurred at 17^oC.