ABSTRACT. Females of Wyeomyia smithii (Coquillett) mate immediately following adult eclosion. Those from Massachusetts emerge with their ovaries at a more advanced stage of development than those from Florida. Decapitation of teneral females prevented the completion of ovarian maturation among individuals from Florida but not from Massachusetts. Application of a JH analogue to Florida pupae induced precocious ovarian development and compensated for decapitation of adults. Juvenile hormone analogue (Altosid) treatment after decapitation at emergence did not stimulate egg development in Florida females beyond the previtellogenic stage (Stage II). Precocious ovarian maturation apparently results from the action in pupae of JH which mediates an early release of a gonotropic factor from the head.