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 timedependent manner, with an IC_50 value of 12.6 μM.