The male accessory gland product that modulates the host-seeking behavior of female Aedes aegypti (L.) mosquitoes was found to be a peptide of 7,600 mw. This peptide also prevented subsequent mating behavior and weakly stimulated oviposition. Neither whole glands nor gland fractions from Anopheles gambiae had any effect on Ae. aegypti females, but those from Aedes albopictus were active.