Bionomic and phylogenetic research on the Tabanidae is hampered by an inadequate knowledge of their immature stages, the larvae and (or) pupae of only 154 of the approximately 2000 world species being known. The present study is an attempt to partially rectify this situation in North America where it is almost as acute as elsewhere.By means of hand-searching or using a specially designed sieve and multiple Berlese funnel, larvae were obtained from 219 wetland habitats of various types in Ontario, New York, Pennsylvania, Vermont, and New Jersey. Many of the larvae were then reared to the adult stage to provide conclusive species identification. During the rearings larval and pupal exuviae were retained and preserved. These together with accurately associated whole preserved larvae and the immatures of several species obtained from other sources were the basis for detailed general descriptions of tabanid larvae and pupae, keys and diagnostic descriptions of these stages as well as descriptive comments on the habitats of the 36 species of Chrysops, 1 Merycomyia, 19 Tabanus, 7 Atylotus, and 18 Hybomitra which are included in the paper. The larvae and pupae of 43 species are described for the first time.In discussing the phylogenetic implications of these larvae and pupae several evolutionary trends and the high degree of concordance between these stages and the adults is pointed out, which lends considerable support to the present classification based solely on adults. The non-alignment of the immatures of some species suggests their improper placement. However, this cannot be proved until the immature stages of many more species covering a broader spectrum of the family are known.Parasitism involving Carinosillus tabanivorus (Hall) (Tachinidae), Villa lateralis Say (Bombyliidae), Diglochis occidentalis Ashm. (Pteromalidae), and Trichopria sp. (Diapriidae) accounted for approximately two per cent of the mortality of reared specimens.