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.Dengue viruses are members of the genus Flavivirus of the family Flaviviridae. There are four serotypes of dengue viruses, DEN-1, DEN-2, DEN-3, and DEN-4. Over the past 20 years, epidemics caused by the four dengue viruses have emerged as one of the major public health problems in tropical and subtropical regions (4,6,18,30). Dengue virus contains a positivesense single-stranded RNA genome. Flanked by two nontranslated regions, there are three structural genes, the capsid (C), precursor membrane (PrM), and envelope (E), at the 5Ј onefourth and seven nonstructural genes at the 3Ј three-fourths (4,14).Dengue virus is transmitted to human by the bite of an infected mosquito. Aedes aegypti is the principle vector involved in the urban transmission cycle (4,10,22). After a female mosquito ingests a blood meal from a patient infected with dengue virus, viral replication is initially found in the posterior midgut of the mosquito and then in the proventriculus and in other organ systems. Dengue virus appears in the salivary gland after an extrinsic incubation period of 8 to 12 days, when the mosquito becomes capable of transmitting it to another human host (10,20,22). Following an incubation period of 3 to 14 days, the infected individuals may be asymptomatic or present a mild and self-limited illness, dengue fever (DF), or a severe and potentially life-threatening disease, dengue hemorrhage fever-dengue shock syndrome (DHF-DSS) (4, 30).The genetic stability of arboviruses that replicate alternately in the vertebrate and arthropod hosts has been well documented previously (13,27,28). In the case of DEN-3 virus, it has been reported that the amino acid similarity of the PrM/E proteins was more than 95% over a 36-year period (13). Previously, it was reported that dengue virus, like other RNA viruses, is present as a population of closely related sequences, the quasispecies, in the human host (2,3,8,16,24,25). The extent of sequence variation of dengue virus in the mosquito vector and how the quasi...