<jats:title>Abstract</jats:title> <jats:p>A new cytotoxic and linear peptide (IC<jats:sub>50</jats:sub> 460 nM to NCI-H460 human lung tumor cells) was isolated from the marine cyanobacterium <jats:italic>Geitlerinema</jats:italic> sp. The planar structure of mitsoamide was deduced by 1D and 2D NMR experiments in combination with MS analyses. The structure of mitsoamide contains an unusual polyketide unit (3,7-dimethoxy-5-methyl-nonanedioic acid, DMNA), incorporates a homolysine (HomoLys) residue and possesses a highly unusual piperidine aminal moiety. The configurations of the relatively common amino acids present in mitsoamide (Ala, Ile, <jats:italic>N</jats:italic>-Me-Ile, Phe, Val) were determined by chiral HPLC analysis of the acid hydrolysate.