The nematophagous fungus Arthrobotrys oligospora Fres. can live saprophytically as well as predatorily. As a predator it forms sticky reticulate traps in the presence of living nematodes which enable it to kill and consume the animals. In laboratory experiments the nutrient acquisition of the fungus was tested on agar media with various N- and C-sources in different concentrations. The intensity of predacious response was determined by counting the traps formed on the agar surface after induction by nematodes. Predacious activity (PA) served as a unit of the strength of nematophagous response. PA was defined as the number of traps/100 mm2. In contrast to carnivorous higher plants, which consume insects in order to compensate their N-deficit, the fungus seems to kill nematodes to get both, N as well as C. If the agar substrate contains a certain concentration of N and C, the fungus does not form traps and lives as a saprophyte. The level which determines the switch towards pure saprophytism under laboratory conditions ranges around 0.12 M C at a N-concentration of 0.05 M. If no nitrogen is added to the medium the fungus forms nematode-induced traps even in the most concentrated C-media. On N-media without any carbon source the fungus shows a much stronger PA compared with N-free media. Furthermore, more chlamydospores were formed on N-media.