Abstract To determine the principal vectors of Lyme disease in North China, the capability of Dermacentor silvarum and D. nuttalli to transmitLyme spirochetes transstadially was estimated under laboratory conditions. Spirochete\|free larval ticks were found to acquire Borrelia garinii if allowed to feed on infected KM mice. PCR tests indicated that 50% of such larvae subsequently became infected by B. garinii. However, cultivation revealed that only about 20.0% of these larva harbored live spirochetes. Spirochetes were detectable in D. silvarum and D. nuttalli larvae up to 8 days after ingestion but remained alive for only 2 days after ingestion. All engorged larvae no longer contained live spirochetes by the time they reached the nymph stage. PCR and cultivation indicated that both newly emerged nymphs and KM mice parasitised by these nymphs were negative for B. garinii. Spirochete-free nymph ticks could also acquire B. garinii by feeding on infected KM mice. PCR indicated that 50% of such nymphs became infected. Cultivation of spirochetes from engorged nymphs indicated infection rates of 33.3% and 60.0% respectively. Spirochetes were detectible for between 4 and 6 days but remained alive for only 2-3 days after ingestion. None of the engorged nymphs contained live spirochetes by the time they reached the adult stage. After molting, neither newly emerged adults or KM mice parasitized by them were found to be infected. The same tick species collected from different sites were similarly incapable of maintaining live Lyme spirochetes. We conclude that D. silvarum and D. nuttalli are unable to transmit infectious B. garinii to subsequent developmental stages. The presence of spirochetes in D. silvarum and D. nuttalli may be tied to co-feeding with Ixodes persulcatus.