Abstract
Larvae of freshwater salmonid nematodes, Sterliadochona ephemeridarum (third-stage) (Spirurida: Habronematodea: Cystidicolidae) and Rhabdochona onchorhynchi (third- and fourth-stage) (Spirurida: Thelazioidea: Rhabdochonidae), were described from mayfly larvae, Ephemera japonica, collected in mountain rivers in central Japan. E. japonica larvae proved natural intermediate hosts for these two nematode species for the first time. Changes in number and shape of teeth of the prostom during development from the third-stage larva to the adult in R. oncorhynchi were briefly discussed.