The present study was planned to use the technique of Polymerase chain reaction (PCR) to detect the virus of dengue fever in mosquito females of Aedes aegypti (L.) and Ae. caspius (Pallas). The results indicated that all samples tested for virus of dengue were found  negative (-) except one sample of  Ae. aegypti which was positive(+). On the other hand, susceptibility levels of mosquito larvae of field strains of Ae. aegypti, which is the vector of dengue fever, to some insecticides currently in use in Jeddah governorate were determined. The LC₅₀ values (concentration which kills 50% of mosquito larvae) were taken in consideration. Mosquito larvae of Ae. aegypti proved to be more susceptible to Bactilarvae (0.012 ppm) than Safroten (0.020 ppm), Temephos (0.032ppm), Solfac (.039 ppm) and Icon (0.047ppm), respectively.