Spectroscopic data define the structures of three new antibiotics, 4', 5'-dihydro-oligosporon (4), hydroxyoligosporon (5) and 10', H'-epoxy oligosporon (6) from the nematode-trapping fungus Arthrobotrys oligospora, and confirm the structures of the recently reported antibiotics oligosporon (1) and oligosporol B (3). The absolute configuration of the substituted 7-oxabicyclo[4.1.0]hept-3-ene nucleus of these metabolites is determined by circular dichroic spectroscopy. Oligosporon (1) and its dihydro-derivative (4) represent the second and most complex structural type of nematocidal metabolite to be characterised from cultures of nematophagous fungi.