This paper on the Trypetidae of New England forms the major portion of a thesis for the degree of Doctor of Philosophy at the Massachusetts Agricultural College. It was originally intended to fix the relationships of the genera, establishing them upon a firmer foundation by using characters which would vary only between the genera, but owing to the difficulty of getting sufficient material this plan was abandoned. An attempt has been made, instead, to more carefully describe the structural characters of the flies, with the hope that some benefit may be derived by future workers of the group.