Three potent cancer cell cytotoxins, piperazimycins A-C (1-3), have been isolated from the fermentation broth of a Streptomyces sp., cultivated from marine sediments near the island of Guam. The structures of these cyclic hexadepsipeptides were assigned by a combination of spectral, chemical, and crystallographic methods. The piperazimycins are composed of rare amino acids, including hydroxyacetic acid, alpha-methylserine, gamma-hydroxypiperazic acid, and gamma-chloropiperazic acid. The novel amino acid residues 2-amino-8-methyl-4,6-nonadienoic acid and 2-amino-8-methyl-4,6-decadienoic acid were found as components of piperazimycins A and C, respectively. When screened in the National Cancer Institute's 60 cancer cell line panel, piperazimycin A exhibited potent in vitro cytotoxicity toward multiple tumor cell lines with a mean GI50 of 100 nM.