Sperm transfer through the epididymis, a prerequisite for insemination of cat fleas, Ctenocephalides felis (Bouché), was stimulated by exposure of unfed male fleas to juvenile hormone III residues for 3 d at 25°C or exposure of unfed fleas to 37°C for 6 d. Sperm transfer was completed at least three times faster in unfed males held at 37°C than in those held at 25°C. Although percentage sperm transfer in fleas fed water or 0.15 M saline at 37°C was not significantly increased over that of unfed fleas, a significantly greater percentage of blood-fed males completed sperm transfer at 2, 3, and 6 d. At least two factors influenced insemination: exposure of fleas to host body temperature and amount of food consumption. When blood-fed males and females were paired and fed 0.15 M saline, 0% were inseminated at 25°C versus 35% at 37°C. Because percentage insemination did not increase in blood-fed males and females that were paired and fed 0.15 M saline at 37°C for an additional 48 h, continuous bloodfeeding appeared to be required for maximal rates of mating and insemination. Furthermore, no females were inseminated when blood-fed males and females were paired at 37°C and starved. Treatment of unfed fleas with juvenile hormone III did not substitute for bloodfeeding in stimulating mating and insemination; when blood-fed males were paired with JH III-treated females and vice versa and fed 0.15 M saline at 37°C, 0% were inseminated. However, when fleas were fed 0.15 M saline and exposed to 1,250 ppm juvenile hormone III or fed whole blood and exposed to 12.5, 125, or 1,250 ppm juvenile hormone III, percent insemination was significantly increased in comparison to the controls. Therefore, juvenile hormone secretion in blood-fed fleas may regulate mating success indirectly by stimulating sperm transfer.