The present paper is the result of an attempt to segregate the species confused in collections under the name Dolopius lateralis Esch. and to determine the characters of the genus Dalopius Esch. (Dolopius auct.). It is based on material in the Canadian National Collection supplemented by specimens from the collections of Messrs. Ralph Hopping, G. Stace Smith, J. B. Wallis, F. S. Carr, L. J. Milne, and E. H. Strickland. More than one thousand specimens of Dalopius, mostly from Canadian localities, have been available for study. I have recognized forty-two species in this material.