Craniometric characteristics of the mouse opossum, Marmosa robinsoni, in Venezuela were analyzed with respect to sex, climate, and macrohabitat. We assessed patterns of phenotypic variation by comparing 6 samples established on the basis of geographic and taxonomic criteria. For most parameters, males were larger than females, and there was geographic variation in skull size that was not related to geoclimatic factors but to the type of vegetation. Specimens inhabiting agricultural lands and disturbed forests were larger than those from cloud and gallery forests; the latter generally was associated with savannas. We suggest that large skull size is related to the higher productivity of secondary-growth vegetation in anthropogenic areas compared with mature forests. We conclude that the specimens studied should be considered as a single subspecies, which corresponds to M. r. robinsoni.