A series of experiments on corn meal agar was carried out to evaluate the efficacy of the nematode-trapping fungus Duddingtonia flagrans in different abiotic and biotic conditions which occur in cow pats. Above a concentration of 50 parasitic larvae (L3) cm–2 the fungus produced a maximum of between 500 and 600 nets cm–2 at 20°C in 2 days on the surface of corn meal agar. There were no differences in the trap-producing capacity of three strains of D. flagrans (CIII4, CI3 and Trol A). On agar at 30° and 20°C, the fungus responded to Cooperia oncophora L3 very quickly producing a maximum of trapping nets 1 day after induction. At 10°C, traps were produced slowly starting on day 4 after induction and continued over the following week. Duddingtonia flagrans (CI3) grew at a normal rate at least down to an oxygen concentration of 6 vol.% O2, but it did not grow anaerobically. On agar, D. flagrans (CI3) did not produce trapping nets in an anaerobic atmosphere. Moreover, C. oncophora L3 stopped migration under anaerobic conditions. When the fungal cultures were transferred to a normal aerobic atmosphere, after 1 and 2 weeks under anaerobic conditions, the C. oncophora L3 resumed migrating on the agar and, in response, D. flagrans produced traps in the same amount as when it had not been under anaerobic stress. Under microaerophilic conditions (6 vol.% O2) D. flagrans was able to grow, but the C. oncophora L3 were not able to induce trapping nets in D. flagrans (Trol A) because of larval immobility. But, as under anaerobic conditions, the fungus could return to a nematode-trapping state when transferred to a normal aerobic atmosphere within 1 or 2 weeks if migrating nematodes were present. Under natural conditions in the cow pat it is expected that the fungus will be ready to attack parasitic larvae, when the oxygen tension increases as a result of, for example the activity of the coprophilic fauna. Artificial light giving 3000–3400 Lux on the surface of the agar significantly depressed the growth rate and the production of trapping nets in D. flagrans (CI3). On agar, D. flagrans (CI3) could grow and produce trapping nets at pH levels of 6.3 to 9.3. Net-production has its optimum between pH 7 and 8. On dry faeces mycelial growth was 7–10 mm during a 15 day period while on moist faeces the fungus expanded 15–20 mm during the same period. Based on the parameters investigated, D. flagrans is expected to be especially active in the well aerated surface layer of a cow pat, an area which normally contains a high concentration of infective nematode parasite larvae, but also an area where the temperature can be high and the water content low.