AbstractTwo new Iotonchium species are described from Japan. These species possesses four adult forms; mycetophagous female, infective female, male and insect-parasitic female. Iotonchium laccariae n. sp. is characterised by short body length of all adult forms, right-angled L-shaped spicule with beak-like distal arm, a pair of large papillae anterior to the cloacal opening, the dorso-ventrally flattened head of male and the reproductive features of the parasitic female. The mycetophagous females of I. laccariae n. sp. inhabit the fruiting bodies of four Laccaria spp. Males and infective females were also obtained from the fruiting bodies. The insect-parasitic females of I. laccariae n. sp. inhabit the haemocoel of a fungus gnat, Allodia laccariae. Iotonchium russulae n. sp. is characterised by the long body length of the infective female, parasitic female and mycetophagous female, obtuse-angled L-shaped spicule with dorsally curved slender distal arm, a pair of bifid postcloacal papillae, presence of a mid-ventral postcloacal papilla in the male and reproductive features of the parasitic female. The mycetophagous females, males and infective females of I. russulae n. sp. were obtained from the fruiting bodies of three Russula spp. and two Lactarius spp. The insect-parasitic females of I. russulae n. sp. inhabit the haemocoel of A. bipexa, a species of fungus gnat.