SINCE Karlson1 isolated α-ecdysone from silkworm pupae, other workers have isolated and identified three more ecdysones—β-ecdysone (crustecdysone)2, 26-hydroxy-β-ecdysone3 and 2-deoxy-β-ecdysone4—in extracts of insects and crustaceans. Additional types of ecdysones will probably be described when other species of arthropods are closely examined, as indicated by the work of Burdette and Bullock5 on silkworm pupae. The presence in one organism of several different ecdysones, differing only slightly in structure and in biological activity, has proved enigmatic. It has been suggested that these compounds are all intermediates in a single ecdysone synthesis degradation scheme6, and Horn et al.7 have suggested that α-ecdysone is the precursor of β-ecdysone (crustecdysone). We now present evidence that this contention is indeed correct; specifically, that both crustaceans and insects are able to convert α-ecdysone to β-ecdysone.