<jats:title>Abstract</jats:title><jats:p>Two new cytochalasans, trichoderones A (<jats:bold>1</jats:bold>) and B (<jats:bold>2</jats:bold>), and three known analogues, aspochalasins D (<jats:bold>3</jats:bold>), J (<jats:bold>4</jats:bold>), and I (<jats:bold>5</jats:bold>), were isolated from the endophytic fungus <jats:italic>Trichoderma gamsii</jats:italic>. Their structures were determined by analysis of their spectroscopic data. The absolute configurations of <jats:bold>1</jats:bold> and <jats:bold>2</jats:bold> were established by quantum chemical electronic circular dichroism calculations. Compounds <jats:bold>3</jats:bold> and <jats:bold>4</jats:bold> displayed cytotoxic activity against the HeLa cell line. Trichoderone A (<jats:bold>1</jats:bold>) possesses an unprecedented 7/6/6/5/5 pentacyclic system, whereas trichoderone B (<jats:bold>2</jats:bold>) contains the rare 6/5/6/6/5 pentacyclic skeleton with a 12‐oxatricyclo [6.3.1.0<jats:sup>2,7</jats:sup>] moiety.