Four species were recognized by MOOJEN (1948) for the subgenus Trinomys, genus Proechimys: P. setosus, P. albispinus, P dimidiatus, and P iheringi. The latter two species are very similar and difficult to distinguish on a morphological basis. Proechimys dimidiatus is a monotypic species restricted in distribution to the state of Rio de Janeiro, whereas six subspecies have been described for P iheringi. Of the six subspecies recognized for P iheringi, P. i. bonafidei and P i. iheringi occur in the state of Rio de Janeiro, and thus can be confused with P dimidiatus. We examined qualitative aspects of the cranium in P dimidiatus, P i. bonafidei, and P i. iheringi to assess the usefulness of these structures to characterize and separate these taxa. Bacular morphology was investigated in P dimidiatus and P. i. iheringi. We also employed standard multivariate morphometric procedures to determine whether the three taxa can be discriminated by quantitative cranial characters. The structure of the incisive foramen, including the shape of the vomerine sheath and the pos-orbital process of zygoma were shown to be important characters to separate P dimidiatus, P. i. bonafidei, and P. i. iheringi. Proechimys dimidiatus and P. i. iheringi also differ in bacular morphology. Discriminant function analysis of 18 cranial metric traits correctly allocated all individuals of P. dimidiatus, P. i. bonafidei, and P i. iheringi to their predicted groups, in spite of substantial variation in cranial dimensions within and among the three taxa.22841795189200