The effect of La Crosse (LAC) virus infection on Aedes triseriatus overwintering success was determined. Eggs from LAC virus transovarially infected (LAC TOT+) and uninfected (LAC TOT-) Ae. triseriatus colonies were induced into diapause, held in natural conditions, and returned to the laboratory at predetermined times for assay of diapause, mortality, and filial infection rates, and to examine viral transcription and replication during diapause. Embryos from the LAC TOT+ colony exhibited greater cumulative mortality (16.7%) than the LAC TOT- eggs (7.3%) throughout the overwintering periods. The increased mortality rate in LAC TOT+ eggs corresponded with a decrease in filial infection rates. Eggs from the LAC TOT+ colony terminated diapause more readily than the LAC TOT- colony. An RNA strand-specific reverse transcriptase-polymerase chain reaction technique was used to monitor viral transcription and replication in mosquito eggs during overwintering, and to compare viral replication in diapausing and nondiapausing embryos. Viral messenger and replicative form RNA were present in eggs in all sample periods, suggesting that some virus replication occurred during diapause.