Proechimys dimidiatus (Guenther) is an echimyid rodent restricted in distribution to the state of Rio de Janeiro, Brazil. This species was regarded by MOOJEN (1948) to lack geographic variation in pelage, skull, and teeth characters and thus considered monotypic. In the present study, morphometric cranial variation was analyzed in population samples of P. dimidiatus by means of univariate and multivariate procedures. The statistical analyses employed in this paper detected among-locality character variation and population differentiation in P. dimidiatus in spite of the existence of significant within-population variation in skull dimensions. Both univariate and multivariate analyses disclosed no clear geographic trend in cranial character variation in P. dimidiatus. The three populations analyzed were shown to be different in the reduced space of canonical variates, although there seems to be no correlation between morphometric differentiation and geographic distance.22541795383390