Juvenile hormone (JH) controls diverse physiological and developmental events including diapause and nutrient metabolism. The focal point of endocrine regulation in adult reproductive diapause is initiated by a halt of JH synthesis. In diapausing females of the mosquito Culex pipiens, the other key molecular event is the signalling pathway from insulin to forkhead of transcription factor (FOXO). We hypothesized that a halt of JH synthesis is related to activation of FOXO, which results in increasing lipid reserves in the fat body at the onset of the diapause programme. In this study, the full-length sequence of the foxo gene in C. pipiens was characterized, and the protein abundance pattern of the foxo gene product was analyzed by immunoblotting and immunohistochemistry. FOXO was much more abundant in the fat body of diapausing females than in the fat body of nondiapausing females; much lower levels were present in other adult tissues. When we topically applied JH III to diapause-destined females, FOXO was suppressed, and fat accumulation was reduced, suggesting an interaction between JH synthesis and FOXO that is critical for expression of the diapause phenotype.