Japanese species of the Mycetophilidae and Sciaridae were reviewed. Ten Palaearctic species of the mycetophilid gnats, Mycomya matilei, M. simulans and M. neodentata, Sciophila dziedzickii and S. interrupta, Leia alternans and L. bilineata, Exechia dorsalis, Trichonta vittata and Phronia willistoni, and eight Palaearctic species of the sciarid gnats, Bradysia subbetuleti and B. brachystyla, Ctenosciara nudata, Phytosciara flavipes, P. ninae and P. ussuriensis, and Sciara humeralis and S. lackschewitzi, were recorded from Japan for the first time. Larval host fungi for the mycetophilid gnats were given. A replacement name, subgenus Merizomma nom. Nov., for the subgenus Chorizomma Sasakawa, 1997, of the genus Lycoriella Frey (Sciaridae) was proposed, because Chorizomma was preoccupied as an arachnid genus name by Simon, 1872.
