The Japanese encephalitis vaccine virus (JE 2-8) and its parent virus (JE SA 14) strains were compared for their ability to infect, to replicate in, and to be transmitted by Culex tritaeniorhynchus mosquitoes. Both viruses replicated in the mosquitoes after intrathoracic infection. The JE 2-8 vaccine was transmitted to 1/36 (3%) of suckling mice bitten by intrathoracically infected mosquitoes; however, the parent JE SA 14 strain was transmitted to 46/46 (100%) of the mice bitten. In oral infection trials, only 4/36 (11%) of the Cx. tritaeniorhynchus mosquitoes ingesting the vaccine JE 2-8 strain became infected, whereas 19/19 (100%) of those ingesting the parent JE SA 14 strain became infected. The vaccine JE 2-8 strain did not revert to virulence during passage in mosquitoes.