An investigation of the sea hare Dolabella auricularia from Papua New Guinea has led to discovery of the new cyclodepsipeptide dolastatin 16 (3) containing two new amino acid units designated dolamethylleuine (Dml) and dolaphenvaline (Dpv). The structural elucidation was achieved by means of high-field (500 MHz) NMR and tandem MS/MS mass spectral interpretations and allowed the assignment of cyclo-(Pro-Dpv-Pro-Dml-O-Lac-Pro-O-Hiv-MeVal). The new depsipeptide exhibited strong inhibition of growth against a variety of human cancer cell lines.