Phylogenetic analyses of cytochrome-b sequence data from 13 of the 15 currently recognized species of New World porcupines were used to test competing taxonomic hypotheses and to explore scenarios of morphological evolution and biogeography. Consistent with previous studies, the monophyly of Erethizontidae (Chaetomys + Erethizon + Coendou) and the monophyly of Erethizontinae (Erethizon + Coendou) were both strongly supported. However, cytochrome-b sequence data provide no support for the reciprocal monophyly of Coendou, "Echinoprocta, " and "Sphiggurus" as those taxa were previously recognized by authors. All of the erethizontid species recognized in recent revisionary work and represented by multiple sequences in this study were recovered as monophyletic groups. Maximum-likelihood (ML) analyses of these data recovered the following phylogeny for 11 species of Coendou: ((melanurus (ichillus (pruinosus + vestitus))) ((spinosus (bicolor + nycthemera)) (prehensilis (mexicanus (quichua + rufescens))))). Ancestral-state reconstructions based on the ML topology suggest that several morphological characters emphasized in past erethizontid classifications (size, nasofrontal sinus inflation, and long fur) have evolved homoplasiously Maximum-likelihood inference of geographic range evolution suggests that the last common ancestor of living erethizontids was a cis-Andean species, and that most subsequent cladogenesis was also cis-Andean; however, at least two trans-Andean dispersal events are plausibly indicated, as well as two separate invasions of Andean landscapes. Among the most remarkable results of this study are almost-identical sequences of Coendou prehensilis from localities spanning 27°of latitude and 25°of longitude; we speculate that a trophic-niche shift might have allowed rapid range expansion of this species, which accounts for almost all known cases of geographic range overlap and sympatry in the genus Coendou.