Halobacillin (1), a new cyclic acylpeptide of the iturin class, has been isolated from cultures of a Bacillus species (culture # CND-914) obtained from a deep-sea sediment core. The structure of the new compound was assigned on the basis of comprehensive NMR studies of 1 and methyl halobacillin (2), and evaluation of the amino acids obtained by acid hydrolysis. The iturins, a class of cyclic acylpeptides produced exclusively by several Bacillus species, are characterized as polar cyclic heptapeptides with lipophilic β-acyloxy or β-amino fatty acid components. Numerous pharmacological properties have been reported for various iturins, including potent antifungal, antibiotic and antitumor activities. In this paper, we report the isolation and structure elucidation of halobacillin, a novel acylpeptide similar to surfactin, one of the most effective biosurfactants known. Halobacillin is produced by a Bacillus species, culture CND-914, isolated from a marine sediment core taken at -124 m near the Guaymas Basin, Mexico. Halobacillin represents the first acylpeptide of the iturin class produced by a marine isolate. Halobacillin showed moderate human cancer cell cytotoxicity, but in contrast to the iturins, no antifungal or antibiotic activity.