Dolastatin I (1), a cyclic hexapeptide containing three kinds of five-membered heterocycles (oxazole, thiazole, and oxazoline), has been isolated from the Japanese sea hare Dolabella auricularia. The gross structure of 1 was elucidated by spectroscopic analysis. The absolute stereostructure of 1 was determined by chiral HPLC analysis of the amino acid components obtained by acid hydrolysis of 1. Dolastatin I (1) exhibited cytotoxicity against HeLa S3 cells with an IC50 value of 12 μg/mL