Transmission rates of La Crosse (LAC) virus observed in Aedes triseriatus females that had engorged on chipmunks with antibody to LAC and had been mated by infected males 5–11 days later (24%, 69/288) were 40% lower than in those fed on chipmunks without antibody (38%, 112/293). Similar results were obtained in three separate trials using males infected 1) by inoculation with prototype LAC virus, 2) transovarially with a field strain, or 3) transovarially with the field strain following passage through a viremic chipmunk. Similar rates were also observed in trials with F2 and F3 progeny of several strains of Ae. triseriatus collected from LAC-endemic and non-endemic areas. Reduction of oral transmission by venereally infected females mated by transovarially infected males following engorgement of antibody in chipmunks or other vertebrates could be important in the natural control of LAC virus, since most adult chipmunks sampled in endemic areas have antibodies neutralizing LAC. Ten-fold higher rates of venereal infection found in females mated by infected males 5 or more days after engorgement on LAC antibody-negative chipmunks than in those mated without prior engorgement extend previous findings of higher rates of transmission after engorgement on laboratory mice to include the natural vertebrate host.