La Crosse (LAC) virus filial infection rates were 0% for 279 first ovarian cycle larvae, 43% for 380 second ovarian cycle larvae, and 58% for 363 third ovarian cycle larvae from orally infected mosquitoes representing 14 Wisconsin populations of Aedes triseriatus. LAC virus was not detected in 72 pools representing 2,250 first ovarian cycle larvae, while 35 pools and 16 pools each containing 30 second and third ovarian cycle larvae, respectively, were all positive for LAC virus. Similar results were obtained when the extrinsic incubation period temperature was 25 degrees C, 27 degrees C, or variable (17, 23, and 29 degrees C for 8 hours each). LAC virus was not detected in 240 second ovarian cycle larvae in which the bloodmeal for the first ovarian cycle was non-infectious. Infection was not detected in 337 first ovarian cycle larvae from female mosquitoes that had been injected intrathoracically with LAC virus concomitantly with receiving a non-infectious bloodmeal. After an extrinsic incubation temperature of 25 degrees C, LAC virus was discovered in dissected mosquito ovarian tissue 7 days postfeeding on an infectious bloodmeal. The epidemiological implications of these findings are discussed.