Five new ecdysteroids, hyousterones A-D (2-5) and abeohyousterone (6), have been isolated from the Antarctic tunicate Synoicum adareanum along with the known ecdysteroid diaulusterol B (1). Hyousterones B (3) and D (5) are unusual ecdysteroids in bearing the 14β-hydroxyl group, and abeohyousterone incorporates the 13(14→8) abeo steroid skeleton, reflecting a rearrangement of the steroid C/D ring system. Abeohyousterone has moderate cytotoxicity toward several cancer cell lines. Hyousterones bearing the 14α-hydroxy group (2 and 4) were weakly cytotoxic, while the 14β-hydroxy hyousterones (3 and 5) were devoid of cytotoxicity. The 14β-hydroxy function may be a thermodynamic pathway to the 13(14→8) abeo steroid skeleton. Hyousterones, abeohyousterone, and diaulusterol B are the first ecdysteroids reported from tunicates. © 2007 American Chemical Society and American Society of Pharmacognosy.