Four novel cyclic undecapeptides, lyngbyazothrins A (1), B (2), C (3), and D (4), were isolated from the cultured Lyngbya sp. 36.91 as binary mixtures (1/2 and 3/4). Their structures were elucidated by analysis of 1D (1H and 13C) and 2D (COSY, TOCSY, ROESY, NOESY, HMQC, and HMBC) NMR spectra, ESIMSMS, ESITOFMS, and amino acid analyses. Three unusual amino acids were present and identified as 4-methoxyhomophenylalanine in 1 and 3, homophenylalanine in 2 and 4, and 3-amino-2,5,7,8-tetrahydroxy-10-methylundecanoic acid (Aound) in all compounds, while 3 and 4 have an additional N-acetyl-N-methyltyrosine unit. The mixture of lyngbyazothrins A (1) and B (2) shows only low antimicrobial activity against Micrococcus flavus, whereas the mixture of lyngbyazothrins C (3) and D (4) was active against Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa, and Serratia marcescens. It seems that the acyl residue at C-5 of the Aound unit plays an important role in antimicrobial activity.