A new phytoecdysteroid, 2-dehydro-3-epi-20-hydroxyecdysone, together with 20-hydroxyecdysone have been isolated by bioassay/RIA-directed HPLC analyses of a methanol extract of the seeds of Froelichia floridana. The structure of the novel ecdysteroid was determined unambiguously by UV, LSIMS, and a combination of 1D and 2D NMR techniques. The biological activity in the Drosophila melanogaster B I I cell bioassay (ED 5 0 = 4.0 x 10 - 7 M) is considerably lower than that of 20-hydroxyecdysone (ED 5 0 = 7.5 x 10 - 9 M).