Physiological amounts of 20-hydroxyecdysone (250 or 1000 pg) injected into female Aedes aegypti mosquitoes had no measurable effect on host-seeking, biting or oviposition behaviors. Larger doses of 0.5 and 1.0 ug reduced the proportion of mosquitoes which engaged in host-seeking and oviposition, but did not affect biting behavior. The host-seeking behavior of ovariectomized mosquitoes receiving a physiological dose of 20-hydroxyecdysone 4-7 hr after a blood meal was not inhibited at 48 hr after the meal. It is suggested that large amounts of the hormone cause a traumatic, nonspecific behavioral inhibition.