In the body cavity of a female specimen of the fly, Sepsis cynipsea L., (Diptera-Cyclorrhapha), caught at this Institute in August, 1930, the writer found eight adult nematode parasites; 3 females and 5 males. The fly was dissected in Ringer's solution in which the worms became actively motile when set free from the host. At the time of dissection it was seen that the fly's ovaries were in an undeveloped state and there is no doubt that this was due to the presence of the parasites within the body cavity. Beyond noting the fact that the worms seemed to be of a mermithid type they were not investigated further at the time but were fixed in hot 70% glycerine alcohol, to which a trace of Nile Blue sulphate solution was added, and were set aside. Although a good number of the flies were dissected during the month of-August no other example was found to be infected with this species of nematode and, as the worms have not been found again since 1930 in casual specimens of the fly captured and dissected here, it seems desirable to describe them since they appear to be new to science. Structurally they bear some resemblance both to Aproctonema entomophagum Keilin, 1917 and to Tetradonema plicans Cobb, 1919 but differ from these species in several anatomical features. Their systematic relationships are discussed in detail later on.