<jats:title>Abstract</jats:title><jats:p>Two new cytochalasans, proxiphomin and protophomin have been isolated from cultures of a <jats:italic>Phoma</jats:italic> species (strain S 298). They are shown to possess the same skeleton as deoxaphomin (<jats:bold>4</jats:bold>), which has been isolated recently. Thus they represent lower states of oxidation of <jats:bold>4</jats:bold>. On the basis of the spectral data, the structures 16‐methyl‐10‐phenyl‐[13]cytochalasa‐6 (7), 13<jats:sup><jats:italic>t</jats:italic></jats:sup>,21<jats:sup><jats:italic>t</jats:italic></jats:sup>‐triene‐1,23‐dione (<jats:bold>1</jats:bold>) and 16‐methyl‐10‐phenyl‐[13]cytochalasa‐5 (6)13<jats:sup><jats:italic>t</jats:italic></jats:sup>,21<jats:sup><jats:italic>t</jats:italic></jats:sup>‐triene‐1,7,23‐trione (<jats:bold>2</jats:bold>) are proposed for proxiphomin and protophomin respectively. On treatment with perchloric acid, substance <jats:bold>1</jats:bold> is isomerized to give 16‐methyl‐10‐phenyl‐10‐pheny‐10‐phenyl‐[13]cytochalasa‐5(6)13<jats:sup><jats:italic>t</jats:italic></jats:sup>,21<jats:sup><jats:italic>t</jats:italic></jats:sup>‐triene‐1,23‐dione (<jats:bold>3</jats:bold>). The assumption that the [13]cytochalasans are potential biogenetic precursors of the 24‐oxa‐[14]catochalasans is discussed.