Prior to the late 1930’s, the familial boundaries of Bibionidae were broad and included taxa now treated as separate families (e.g. scatopsids, pachyneurids, and axymyiids). Although these taxa were eventually removed, leaving Bibionidae with eight genera and approximately 700 species, the lack of putative synapomorphies for this leftover assemblage drove some authors to further subdivide the group (e.g. Hesperinidae and Pleciidae). Because of unclear familial boundaries and at least five conflicting hypotheses of the sister group to bibionids, the position of the family within the nematoceran flies has been unresolved. Cladistic methodology was employed to address these problems. Species exemplars of all bibionid genera (40 taxa) and exemplars of each of the putative sister groups (18 taxa) were included. Two hundred and twelve morphological characters from the adult, larval, pupal, and egg life stages were examined. Results of the analysis indicate that Bibionidae is monophyletic and its sister group is Pachyneuridae. The bibionid subfamilies Hesperininae and Bibioninae are monphyletic and Pleciinae is paraphyletic. All bibionid genera are supported as monophyletic groups except for the genus Bibio.