| Summary: | Eight new cytochalasins <b>1</b>–<b>8</b> and ten known analogs <b>9</b>–<b>18</b> were isolated from the endophytic fungus Phomopsis sp. xz-18. The planar structures of the cytochalasins were determined by HR-ESI-MS and NMR analysis. Compounds <b>1</b>, <b>2</b>, <b>9</b> and <b>10</b> were 5/6/6/7/5-fused pentacyclic cytochalasins; compounds <b>3</b> and <b>4</b> had conjugated diene structures in the macrocycle; and compound <b>6</b> had a β,γ-unsaturated ketone. The absolute configuration of 6 was confirmed for the first time by the octant rule. The acid-free purification process proved that the pentacyclic system was a natural biosynthetic product and not an acid-mediated intramolecular cyclized artifact. The new compounds did not exhibit activities against human cancer cell lines in cytotoxicity bioassays or antipathogenic fungal activity, but compounds <b>1</b>, <b>3</b> and <b>4</b> showed moderate antibacterial activity in disk diffusion assays.
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