Tychius mixtus Hatch is newly synonymized with T. tectus LeConte; T. lamellosus Casey, T. squamosus Hatch and T. intermixtus Hatch are synonymized with T. semisquamoslls LeConte, and T. hirsutus Clark is synonymized with T. soltaui Casey. New locality and host records are given for T. tectus LeConte, T. semisquamosus LeConte, T. sordidus LeConte, T. soltaui Casey, and T. liljebladi Blatchley. Comparison of North American Tyehius with species from Eurasia and Africa indicates that the North American species belong to at least 2 major groups, each of which has a greater number of species in the Old World. Phylogenetic and host plant relationships and distribution patterns indicate that Tyehius arose in the Old World; several independent dispersals from Eurasia have formed the North American fauna. Tyehius sordidus and T. caesius Clark are probably remnants of an old Holarctic fauna associated with the Arcto-Tertiary flora. Tyehius lineellus is probably related to the latter two species but dispersed via Beringia. T. liljebladi Blatchley and T. tectus LeConte are more closely allied to members of the semisquamosus Group; all of these have close relatives in Eurasia. The former 2 have as hosts species of Oxytropis and species of Astragalus which belong to Eurasian groups. They probably arrived in North America after the semisquamoslls Group was already established. T. tectus also has Astragalus hosts which belong to native North American groups. It appears to be extending its geographic and host range at the expense of some semisquamosus Group members. Members of the latter group are known only from species of Astragalus in American groups of from the genus Lotus.