Daphnia magna were exposed for 21 d to the ecdysteroids, 20-hydroxyecdysone (20-E), the accepted molting hormone, and ponasterone A (PoA), an ecdysteroid found in some crustaceans and many plants. Daphnids were monitored for alterations in molting, fecundity, and survival time. The 20-E elicited no significant effects on molting frequency, and its significant effects on reproduction were only at concentrations (260 nM) associated with premature death caused by incomplete ecdysis. We also examined PoA, which has been reported to have 10x higher affinity for the ecdysone receptor than 20-E. Ponasterone A elicited effects similar to those of 20-E at approximately 10x lower concentrations. This suggests that affinity for the receptor is the major parameter determining activity in vitro and in vivo and that differences in metabolism and elimination in vivo were not significant. The effects of PoA on daphnids mimicked those of 20-E except PoA reduced fecundity in the second generation and 20-E had no effect. Last, both exogenous 20-E and PoA show similar effects, including premature death associated with incomplete ecdysis, and the overall difference in toxicity is mostly likely due to receptor affinity.