The La Crosse (LAC) virus infection rate of Aedes triseriatus larvae that ingest LAC virus does not appear to be increased by concomitant infection of larvae by the gregarine parasite, Ascocystis barretti. Infection rates ranged only from 0–2.6% in adult Ae. triseriatus reared from groups of A. barretti-infected larvae that had ingested LAC virus (California encephalitis group) at dosages of 2.0–7.7 log10 SMICLD50/ml. Females resulting from orally infected larvae transmitted LAC virus to suckling mice. Larvae that were infected with A. barretti and deveoured carcasses of adult mosquitoes containing 4.7 log10 SMICLD50/ml failed to become infected. A. barretti spores developing in transovarially infected mosquitoes did not harbor LAC virus; thus, A. barretti does not appear to be a mechanism for virus dispersal.