For the second time, the authors have found a new entomophagous nematode parasitic in Syrphidae. The first species, which was described as Syrphonematidae nov. fam., is living in the duct of various aphidophagous species of Syrphidae. This one has been found in the hemocoel of adults of Helophilus trivittatus Fabricius and H. pendulus Linneaus (Eristalinae with aquatic rat tailed larvae). It seems to belong to the genus Iotonchium (Allantonematidae), which was known as yet only by the free-living stages. This work could bring the proof that the genus Iotonchium is really parasite of insect as it was supposed by T. GOODEY and J. B. GOODEY. The three distinct forms which are living in the body cavity of the host are briefly described. The life cycle, which differs from those of all other Allantonematidae, is discussed. This species requires further studies, especially on the free-living stages which have to be obtained for the comparative study of the morphology of the various forms and for life history.