A new cell cycle inhibitor, nidurufin (1), was isolated from the marine-derived fungus Penicillium flavidorsum SHK1-27. An evaluation of antitumor activity indicated that 1 induced in vitro cell cycle arrest at $G_2$/M transition in the K562 cell line in a concentration and time-dependent manner, with an $IC_{50}$ value of $12.6{mu}M$.