The effects of the juvenile hormone analog pyriproxyfen (at concentrations of 0.1, 0.5, and 1%) on egg production, number of ovarioles, and length of oocytes were examined in queens of the Pharaoh ant Monomorium pharaonis (L.). Pyriproxyfen significantly reduced egg production in queens from week 3 onwards. Queens that were exposed to 1% pyriproxyfen stopped producing eggs at week 8. After 8 wk, ovaries were dissected from all queens, and the number of ovarioles and the length of the largest oocytes were recorded. The ovaries of queens in treated colonies were smaller than those in untreated queens, and the number of ovarioles in the ovaries was significantly lower in all pyriproxyfen-treated queens. Queens treated with the highest concentrations of pyriproxyfen tended to have significantly shorter oocytes than untreated queens. Histological studies of the ovaries revealed that pyriproxyfen caused vacuolation in the ovarioles, thickening of the tunica propria, development of small eggs, and underdevelopment of nurse cells and the follicular epithelium. Exposure to pyriproxyfen reduced egg production and induced severe morphological changes in the ovaries of queens, and the effects increased with increased concentration of pyriproxyfen.