The primary structures of unknown cyclic peptides produced by a Bacillus strain have been determined by fast atom bombardment (FAB) mass spectrometry, which has established the peptides as a new family of the iturin group antibiotics. FAB mass spectra of the intact peptides gave the immonium ions characteristic of constituent amino acids which made it possible to distinguish Asn from Asp and Glu from Gln. Collision-induced dissociation (CID) spectra of protonated molecules provided complete information on the connectivity of amino acid residues but did not reveal the direction of peptide bonding, while those obtained for fragment ions allowed us to make a discrimination between the correct sequence and its retro sequence. The amino acid sequences derived are c(Thr-Xaa-Asn-Tyr-Asn-Ser-Glu-Ser) (Xaa: C14 or C15 beta-amino acid) which are closely related to that of bacillomycin L. The structure is confirmed by the FAB mass spectra of the partial acid hydrolysate and the peptide mixture obtained from its single-step Edman degradation. Fragmentation processes involved in the CID spectra of the cyclic peptides are discussed based on the established amino acid sequence.