In anautogenous mosquitoes, vitellogenesis, which includes production of yolk protein precursors, requires blood feeding. Consequently, mosquitoes transmit many diseases. Understanding the molecular mechanisms of vitellogenesis regulation will contribute significantly to vector control strategies. Newly emerged Aedes aegypti females require 3 days before becoming competent to activate vitellogenesis in response to a blood-meal-initiated, elevated titer of 20-hydroxyecdysone (20E). An orphan nuclear receptor gene β FTZ-F1 is transcribed in the fat body of newly emerged mosquito females; however, the βFTZ-F1 protein is only found 3 days later. Dramatically increased titer of the juvenile hormone III (JH III) is essential for the acquisition of 20E competence. In vitro fat body culture experiments have shown that βFTZ-F1 protein appears after exposure to JH III. Injection of double-stranded RNA complementary to β FTZ-F1 into newly emerged females attenuated expression of the early genes EcR-B, E74B , and E75A and the target YPP gene Vg , in response to a blood meal. Thus, βFTZ-F1 is indeed the factor defining the acquisition of competence to 20E in the mosquito fat body. Moreover, this is achieved through JH III-mediated posttranscriptional control of β FTZ-F1 .