All stages of Demacentor andersoni (Stiles) ticks were infected by feeding on rabbits injected intravenously with large doses of Powassan virus 48 hours after the ticks became attached. Viremia titers at least 102.5 mouse LD50 per milliliter were required to establish infection in 1 to 5% of D. andersoni ticks whereas Ixodes pacificus (Cooley and Kohls) ticks required 100-fold higher titers of viremia to achieve infection. Transstadial transfer of Powassan virus was observed through ecdysis of larvae to nymphs and from nymphs to adults. Transovarial transfer was not detected. Transmission of Powassan virus to a guinea pig was demonstrated by the bite of D. andersoni nymphs which were infected by ingestion of viremic blood by their antecedent larvae 8 weeks before.