LaCrosse (LAC) virus (California encephalitis group) was isolated from adult Aedes triseriatus mosquitoes that were collected as eggs from ovitraps placed in Peoria and Cook Counties, Illinois during June-September 1979. Because LAC virus is transmitted transovarially, this technique is very useful in identifying areas of LAC transmission without searching for natural or artificial oviposition sites or collecting adults attracted to human volunteers.