Various carbon and nitrogen sources were evaluated to develop an inexpensive medium for the mass production of nematode-trapping fungus, Arthrobotrys oligospora. A. oligospora utilized a wide range of carbon and nitrogen sources. Liquid fermentation system containing sucrose and yeast extract in the ratio of 5:1 was proposed. Maximum yield of 11 g (dry weight)/liter of medium was obtained in 50 h. Production of large amount of nematode-trapping fungi in liquid culture may facilitate further evaluation of these organisms as biological control agents.