1. A detailed morphological study has been made of certain nematodes occurring in the basidiomycetous fungi, Entoloma rhodipolium, Pleurotus corticalus, P. ostreatus, Hygrophorus virgineus and Tricholoma cunifolium.
2. From the first three of these, males and females of two species of eelworms have been obtained which are placed in the genus Iotonchium Cobb, 1920. One of these is Iotonchium fungorum (Butschli, 1878) n. comb., originally described by Butschli under the name of Tylenchus fungorum', the other is a new species which is named I. bifurcatum n. sp.
3. The males of both species have peculiar lobed, dorso-ventrally flattened heads and a poorly developed mouth spear. The bursa is very large, the spicules have posterior prolongations which arc extruded through the cloaca and ventral post-anal papillae are present. A gubernaculum is absent.