<jats:title>Abstract</jats:title><jats:p>The cyclic dodecapeptides tychonamide A (<jats:bold>1</jats:bold>) and B (<jats:bold>2</jats:bold>) isolated from the methanolic extract of the cyanobacterium <jats:italic>Tychonema</jats:italic> sp. contain the novel β‐amino acid 3‐amino‐2,5,7‐trihydroxy‐8‐phenyloctanoic acid (Atpoa). Compounds <jats:bold>1</jats:bold> and <jats:bold>2</jats:bold> have cytotoxic activity towards cancer cell linesin a monolayer assay with mean IC<jats:sub>50</jats:sub> values of 0.9 and 3.3 μg mL<jats:sup>–1</jats:sup>, respectively. Compound <jats:bold>2</jats:bold> was shown to be active against tumor cell suspensions derived fromsolid tumor xenografts in a clonogenic assay (mean IC<jats:sub>50</jats:sub> 2.4 μg mL<jats:sup>–1</jats:sup>). Additionally, antiprotozoal activity was observed (<jats:italic>Trypanosoma b. rhodesiense</jats:italic>, IC<jats:sub>50</jats:sub> 0.1 μg mL<jats:sup>–1</jats:sup>) for compound <jats:bold>1</jats:bold>. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)