During the years 1975-1979 a total of 86 arbovirus isolations were made from 3030 pools of mosquitoes collected from various locations in Manitoba. Most of the virus isolations were western equine encephalomyelitis (WEE). The main vector of WEE Manitoba is Culex tarsalis Coquillett; however, WEE virus was also isolated from other mosquito species. This is the first report of WEE virus isolation from Aedes vexans (Meigen) and Culiseta inornata (Williston) in Manitoba. As far as we know, these are the first isolations anywhere of WEE from Coquillettidia perturbans (Walker), Ae. pionips Dyar and Anopheles earlei Vargas. Cq. perturbans and An. earlei may be important vectors of WEE where they make up a significant proportion of the mosquito population. We report here the first isolation of Cache Valley Virus (CVV) and Hart Park Flanders virus (HPF) from mosquitoes in Manitoba.