Description:
<jats:title>Abstract</jats:title><jats:p>A diplogastrid nematode was isolated from a dung beetle, <jats:italic>Onthophagus</jats:italic> sp., collected from a rotten mushroom in Kyoto, Japan. The species is characterised by its cheilostomatal shape, separated into 12 narrow plates (rugae), deep stegostom, large ellipsoidal amphids, conical female tail and characteristic <jats:italic>receptaculum seminis</jats:italic> in the female. Based on its phylogenetic status and stomatal composition, the species is typologically similar to two other diplogastrid genera, <jats:italic>Neodiplogaster</jats:italic> and <jats:italic>Mononchoides</jats:italic>. The species can be distinguished from these two genera by the size and shape of the amphid (small pore in <jats:italic>Neodiplogaster</jats:italic>), female tail shape (long and filiform in <jats:italic>Mononchoides</jats:italic>) and presence of <jats:italic>receptaculum seminis</jats:italic> (absence in the two nominal genera), and is described as a monotypic member of a new genus, <jats:italic>Onthodiplogaster japonica</jats:italic> n. gen., n. sp. Observation of feeding behaviour suggested that <jats:italic>O. japonica</jats:italic> n. gen., n. sp. does not show clear stomatal dimorphism or polymorphism, which is found in its close relatives, but the species can feed on nematodes (predation), fungi and bacteria. This monomorphic omnivory possibly represents its habitat of dung and other rotten materials, where the environment is biologically divergent, and its condition changes rapidly.</jats:p>