A new hTopo I isomerase inhibitor, (+)-3,3',7,7',8,8'-hexahydroxy-5,5'-dimethylbianthraquinone (2240A), was isolated from the mangrove endophytic fungus no. 2240 collected from an estuarine mangrove at the South China Sea coast. Its structure was elucidated by spectral analyses including two-dimensional NMR, HR-EI-MS, IR, and UV. The hTopo I isomerase inhibition experiment showed that 2240A (1) possessed strong inhibiting activity. When its inhibition concentration was 4.65 micromol/l, its percent inhibition rate was 59.1%, while the lowest inhibition concentration of the positive control camptothecin was 1.00 x 10(3) micromol/l.