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Two epizootic strains of Venezuelan encephalitis (VE) virus from Central America and Mexico were transmitted by a colonized epizootic vector mosquito, Aedes taeniorhynchus, at higher rates than were two enzootic strains when the mosquitoes were infected by intrathoracic inoculation or feeding of virus. Differences in transmission rates also occurred with colonized Aedes aegypti, but were less marked. Following intrathoracic inoculation of A. taeniorhynchus or A. aegypti, epizootic strains grew to slightly higher concentrations in the mosquitoes than did enzootic strains. Intestinal thresholds of infection for A. taeniorhynchus were slightly lower for epizootic than for enzootic virus strains, but were essentially equal for A. aegypti. Only a small percentage of individual Culex pipiens quinquefasciatus mosquitoes supported growth of epizootic VE virus, and only 1 of 6 tested C. p. quinquefasciatus transmitted virus by bite. Thus, transmission and growth of virus in these Aedes mosquitoes distinguished between these epizootic and enzootic strains of VE virus.