The Indian Ocean sea hare Dolabella auricularia has been found to contain a strongly cytostatic depsipeptide constituent designated dolastatin 15. The unusual depsipeptide was found to markedly inhibit growth of the P388 lymphocytic leukemia cell line with ED,, = 0.0024 pg/mL. The structure was elucidated by employing primarily high-field (400-MHz) 2D NMR and high-resolution mass spectrometry.