The structure of bacillomycin L, an antifungal agent isolated from the culture medium of a strain of Bacillus subtilis, has been investigated. The peptide moiety contains one mole each of D-aspartic acid, L-aspartic acid, L-glutamine, L-serine, D-serine, L-threonine, and D-tyrosine. The lipid moiety is a mixture of 3-amino-12-methyltridecanoic acid (46%), 3-amino-12-methyltetradecanoic acid (38%, 3-amino-14-methylpentadecanoic acid (11%), and two minor homologues. The peptide sequence and the cyclic structure were determined by structural analysis of the peptides obtained by mild acid hydrolysis. The molecular weight was determined by the thermoosmotic method; this showed that bacillomycin L has a monomeric structure which is given in Formula 1.