A cladistic analysis of the genus Karos Goodnight & Goodnight, 1944, was performed using morphological data of the somatic and male genitalia characters. The analysis included 23 terminal taxa, including nine of the 11 described species of the genus plus nine new species according to the previous generic diagnosis and five species as outgroups. According to the topologies obtained by parsimony analyses, the genus is a paraphyletic assemblage, referred here as the Karos genus-group. Therefore, the genus Karos is rediagnosed here and now includes seven species: Karos barbarikos Goodnight & Goodnight, 1944 (type), Karos parvus Goodnight & Goodnight, 1971, Karos projectus Goodnight & Goodnight, 1971, Karos hexasetosus sp. nov., Karos monjarazi sp. nov., Karos singularis sp. nov., and Karos tersum sp. nov. The genera Monterella Goodnight & Goodnight, 1944, Montabunus Goodnight & Goodnight, 1945, Chapulobunus Goodnight & Goodnight, 1946, and Potosa Goodnight & Goodnight, 1947 are revalidated, rediagnosed, their respective type species are redescribed and the following species are described: Chapulobunus poblano sp. nov. and Potosa reddelli sp. nov. The genera Crettaros gen. nov., Huasteca gen. nov., and Mictlana gen. nov., and the following species are described: Crettaros santibanezi sp. nov. (type), Crettaros valdezi sp. nov., and Huasteca silhavyi sp. nov. The following new combinations are proposed: Huasteca gratiosa (Goodnight & Goodnight, 1971) comb. nov. (type), Huasteca rugosa (Goodnight & Goodnight, 1971) comb. nov. and Mictlana inops (Goodnight & Goodnight, 1971) comb. nov. (type). Karos brignolii Silhavý, 1974, is considered a junior synonym of Huasteca rugosa. Finally, ‘Karos’ depressus Goodnight & Goodnight, 1971 is considered incertae sedis until adult males can be studied. Diagnoses of the Karos and Paramitraceras genus-groups, and an identification key to the eight genera and 19 species of the former are provided. © 2015 The Linnean Society of London