The predaceous fungus Arthrobotrys dactyloides forms ring-like traps which constrict and mechanically catch nematodes that have entered them. We have found that the fast death of the captured prey is caused by ammonia produced by the fungus. We have also devised a semiquantitative method for the detection of nematode-attracting substances and have shown that A. dactyloides produces compounds of this type, one of which is carbon dioxide.