Specimens of Proechimys from 12 localities in Bolivia, Brazil, Colombia, and Peru, were studied to assess the degree of variation in bacular, cranial, and dental morphology, and pelage. Coefficients of variation and Student's t values were computed for selected cranial measurements to evaluate intra- and interpopulation variation. Studies on tooth wear demonstrated considerable variation in dental occlusal pattern and suggested that taxonomic assignment based entirely on dental morphology may be misleading. Descriptions and illustrations of bacula from most localities indicate that these bones are highly variable in overall dimensions, but have common structural features at a particular locality. Most specimens are tentatively referred to Proechimys guyannensis, although P. canicollis, P. quadruplicatus, and P. steerei were also represented. Proechimys guyannensis is regarded as a highly variable species. It is suggested that subspecific delimitation may not adequately express the variation observable in this species.
