Mosquitoes were collected in Alberta during the spring, summer, and early fall of 1965. Western encephalitis (WE) virus was isolated from pools collected between August 11 and August 25.Two positive pools, one ofAedes vexans(Meigen) and one ofCuliseta inornata(Williston) were collected by sweep nets in irrigated areas of southern Alberta near places where horses were reported sick with WE virus. Seven other positive pools, three ofC.inornataand four ofCulex tarsalisCoquillett, were collected in southern Alberta from mammalian burrows, which are hibernating habitats forC.inornata,C.tarsalis, andAnopheles earleiVargas. This indicates thatC.tarsalisandC.inornatacan enter hibernation infected with WE virus. None of theA.earleipools tested were positive for WE virus.About 50% of theC.inornataand 9% of theC.tarsaliscollected from mammalian burrows were blood-engorged at the time of collection. None of theA.earleicollected from mammalian burrows were blood-fed.