Individuals of the free-living nematode Panagrellus redivivus were infected with conidia of the endoparasitic nematophagous fungus Drechmeria coniospora. At different stages of infection the nematodes were added to an agar-plate culture of the predatory fungus Arthrobotrys oligospora which contained traps. Initially, nematodes infected with D. coniospora were invariably captured, penetrated and colonized by A. oligospora. Then both fungi used the nematode contents for development of hyphal biomass outside the nematode. But inside nematodes in which infection by D. coniospora was advanced development of A. oligospora was markedly hampered. In dual-infected nematodes (colonized from opposite ends), the hyphae of A. oligospora were often observed to be dead or degenerate when in close proximity to living mycelium of D. coniospora, indicating that the latter acts as an antagonist against A. oligospora under these conditions. The interaction between trophic hyphae of D. coniospora and A. oligospora involved thickening of the cell wall of A. oligospora at the sites of contact and degradation of the hyphal compartments and shrivelling of the cell wall of A. oligospora. The observed interactions are discussed in relation to mycoparasitism and possible ecological function. © 1994, British Mycological Society. All rights reserved.