Dengue virus is an arbovirus that replicates alternately in the mosquito vector and human host. We investigated sequences of dengue type 3 virus in naturally infected Aedes aegypti mosquitoes and in eight patients from the same outbreak and reported that the extent of sequence variation seen with the mosquitoes was generally lower than that seen with the patients (mean diversity, 0.21 versus 0.38% and 0.09 versus 0.23% for the envelope [E] and capsid [C] genes, respectively). This was further verified with five experimentally infected mosquitoes (mean diversity, 0.09 and 0.10% for the E and C genes, respectively). Examination of the quasispecies structures of the E sequences of the mosquitoes and of the patients revealed that the sequences of the major variants were the same, suggesting that the major variant was transmitted. These findings support our hypothesis that mosquitoes contribute to the evolutionary conservation of dengue virus by maintaining a more homogenous viral population and a dominant variant during transmission.