A new species of the diplogastrid genus Oigolaimella is described in colonies of termites belonging to the genus Reticulitermes from Corsica (France) and USA. Oigolaimella attenuata n. sp. males can be recognised by the conspicuous length of the ventral unkeeled part of the otherwise keeled gubernaculum and, in contrast to the other members of the genus, the fact that the lateral field of both adult stages is marked by a single line. A diagnostic key for the five species of Oigolaimella is presented. Life cycle and development, including spermiogenesis, sperm transfer, sperm competition and fertilisation are described in detail. Some interesting aberrations of the reproductive system are documented. The new species uses the preoral cavities of the termites for internal phoresis and is associated with non-pathogenic gut-inhabiting flagellates of the taxon Kinetoplastida. The heads of 76 of 117 examined termites were infested with dauer juveniles of O. attenuata n. sp. with an average of 6.4 nematodes per termite. Six additional nematode species were isolated from the bodies of the termites, particularly a species of Pristionchus and, for the first time, Halicephalobus sp., Mesorhabditis spiculigera and Rhabditella axei. Rhabpanus ossiculum was isolated from termite-inhabited wood from Corsica, the first such detection in Europe. In the course of our discussion of the literature on termite-associated nematodes, we propose the new combination Pristionchus formosianus (Poinar, Meikle & Mercadier, 2006) n. comb. (=Chroniodiplogaster formosiana).