Both the specific vector Dermacentor silvarum and the nonspecific one Ornithodoros papillipes are shown to become Rickettsia sibirica infected while sucking blood on the infected guinea pigs. Meanwhile, the specific vector D. silvarum is more sensitive to R. sibirica than the nonspecific. O papillipes. The infectivity of D. silvarum and O. papillipes ticks by R. sibirica was found to be dependent on the intensity of guinea pigs' rickettsemia. Transovarial transmission of Rickettsia has been established for the D. silvarum and O. papillipes ticks, but the Rickettsia titer in the infested D. silvarum larvae exceeded that O. papillipes. Survival of D. silvarum ticks infected by various concentrations of the R. sibirica microbial bodies was found to exceed that of O. papillipes.