AbstractMakisterone A (24‐methyl‐20‐hydroxyecdysone) is approximately ten times more potent than 20‐hydroxyecdysone in the stimulation of cuticle synthesis and inhibition of vitellogenesis when injected into 3‐day‐old virgin females ofOncopeltus fasciatus.By day 6 postinjection with an active dose of makisterone A, the new cuticle appears identical in color pattern and morphology to the old adult cuticle, although the insects never undergo ecdysial contractions. Since milkweed bugs are also known to be incapable of dealkylating phytosterols, and makisterone A has been previously identified as the predominant ecdysteroid from embryonated eggs of this species, it is likely that makisterone A is the molting hormone ofO. fasciatusor is closer to the structure of the indigenous hormone than is 20‐hy