Variation in the complete nucleotide sequence of the mitochondrial cytochrome b gene was examined for 32 individuals representing 12 supraspecific taxa of South American rodents of the family Echimyidae (Hystricognathi). Representative genera of four other New World hystricognath families, the Old World porcupine Hystrix, and the myomorph murid rodents Rattus and Mus were used as outgroups in phylogenetic reconstructions. Monophyly of the family Echimyidae is strongly supported, a result fully consistent with existing morphological and paleontological data relative to the taxa examined. However, relationships among most supraspecific taxa within the family are poorly resolved. Poor resolution appears not to result from lack of data, but to a rapid, nearly simultaneous divergence of most Recent taxa. Generic groupings that are moderately to strongly supported include the tree rats of the Brazilian Atlantic Forest (Nelomys) and Amazonia (Echimys, Makalata) and the Amazonian arboreal spiny rats Mesomys and Lonchothrix. However, the two subgenera of the terrestrial spiny rats, Proechimys, do not form a monophyletic unit, and elevation of the Atlantic Forest Trinomys to generic status is supported. The genus Hoplomys is closely related to Proechimys (sensu stricto), a finding supported by other molecular data. © 1996 Academic Press, Inc.
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