Fifteen of 19 isolations of Jamestown Canyon virus from Connecticut mosquitoes tested since 1969 were recovered from Aedes abserratus; the minimum field infection rate (MFIR) for the 10-year period was 1:438. The other 4 strains were from Ae. cantator (2 isolates) (MFIR 1:4024), Ae. vexans (MFIR 1:25353), and Coquillettidia perturbans (MFIR 1:26666). Keystone virus is reported for the first time in New England. One strain was recovered from Ae. aurifer and another from Chrysops obsoletus. The latter isolation is the first published record of this virus from a Tabanid.