Virologic surveillance for dengue through the detection of the prevalent serotype(s) circulating in the human population during inter- and intra-epidemic periods constitutes a reliable sentinel system for dengue outbreaks. We have applied a rapid and sensitive, semi-nested, reverse transcription-polymerase chain reaction (RT-PCR) assay using nonstructural protein 3 gene primers for the type-specific-detection of dengue viruses in artificially infected and in field-caught adult Aedes mosquitoes. In laboratory experiments, the assay was sensitive enough to detect one virus-infected mosquito head in pools of up to 59 uninfected heads. In a prospective field study conducted from April 1995 to July 1996, female adult Ae. aegypti and Ae. albopictus mosquitoes were caught from selected dengue-sensitive areas in Singapore and assayed by RT-PCR. Approximately 20% of 309 mosquito pools were positive for dengue viruses. Of the 23 RT-PCR-positive Ae. aegypti pools (containing 1-17 mosquitoes each), 18 pools (78.3%) were positive for dengue 1 virus. There were 40 RT-PCR-positive Ae. albopictus pools (containing 1-33 mosquitoes each) of which 31 (77.5%) were positive for dengue 1 virus. The predominant virus type responsible for the current dengue epidemic since 1995 was also dengue 1. The geographic locations of the virus-infected mosquitoes correlated with the residences or workplaces of patients within dengue outbreak areas. A total of 43.5% of the positive Ae. aegypti pools and 25.0% of the positive Ae. albopictus pools contained only a single mosquito. Both Aedes species showed similar overall minimum infection rates of 57.6 and 50 per 1,000 mosquitoes. Infected Ae. aegypti were detected as early as six weeks before the start of the dengue outbreaks in 1995 and 1996. However, infected Ae. albopictus appeared later, when the number of cases was increasing. Virologic surveillance by RT-PCR for detecting dengue virus-infected Aedes mosquitoes in the field may serve as an early warning monitoring system for dengue outbreaks.