Transmission of California encephalitis (CE) virus strain R2929 by groups of Aedes vexans (Meigen) mosquitoes to rabbits was accomplished 7 and 9 days after the insects had imbibed an infective blood meal. Aedes aegypti (L.) mosquitoes transmitted virus to newly hatched chickens by biting them 48, 96, and 144 hours after intrathoracic injection.CE vires was found in gut, thorax, legs, and salivary glands of pools of Aedes triseriatus (Say), Aedes canadensis (Theobald), and A. vexans mosquitoes after intrathoracic injection of 101.3 mouse LD50 per 0.003 ml of virus. Salivary glands contained a maximum virus titer of 105.0 mouse LD50 per anatomical unit 5 days after injection. A. aegypti mosquitoes also supported virus growth after intrathoracic injection but yielded higher virus titers (106.3 mouse LD50) in the salivary glands.The infection threshold of A. vexans fed CE virus was 102.0LD50 per insect. Immediately after ingestion of 102.0 LD50 of virus only the gut washings contained virus. Detection of virus was not accomplished again until 4 days later. Average maximum titers of 104.5 LD50 per salivary glands were found after 8 days of extrinsic incubation. The infection threshold of A. aegypti fed CE virus exceeded 104.5 LD50 per insect.New Zealand white rabbits and Leghorn chickens circulated CE virus in their blood, which attained peak titers of 102.5 mouse LD50 per 0.03 ml 48 and 72 hours respectively after subcutaneous inoculation, but weaned mice did not develop viremia. All three animal species produced neutralizing antibodies to CE virus 21 days after inoculation.