<jats:title>Abstract</jats:title><jats:p>A new natural product, named phomopsin A, 1‐(meta‐hydroxyphenyl)‐4‐hydroxy‐3‐isoquinolone (1), together with two known compounds cytochalasin H (2) and glucosylceramide (3), was isolated from the mangrove endophytic fungus <jats:bold><jats:italic>Phomopsis</jats:italic></jats:bold> sp. (ZZF08) obtained from the South China Sea coast. The structures were elucidated by 1D and 2D NMR experiments including COSY, HMQC, and HMBC. According to NMR and single‐crystal X‐ray diffraction, it was found that some assignments about <jats:sup>1</jats:sup>H and <jats:sup>13</jats:sup>C NMR data for cytochalasin H (2) were probably uncorrected in the previous reports. In our cytotoxicity assays, compound 1 showed moderate cytotoxicity toward KB cells with IC<jats:sub>50</jats:sub> at 28.0 µg ml<jats:sup>−1</jats:sup> and KBv200 cells with IC<jats:sub>50</jats:sub> at 16.8 µg ml<jats:sup>−1</jats:sup>, and compound 2 exhibited strong cytotoxicity toward KB cells and KBv200 cells with IC<jats:sub>50</jats:sub> less than 1.25 µg ml<jats:sup>−1</jats:sup>. Copyright © 2008 John Wiley & Sons, Ltd.