We analyzed mandible shape variation of 17 genera belonging to three superfamilies (Cavioidea, Chinchilloidea and Octodontoidea) of South American caviomorph rodents using geometric morphometrics. The relative influence of phylogeny and ecology on this variation was assessed through phylogenetic comparative methods. Most morphological variation was concentrated in condylar, coronoid and angular processes, and the diastema. Features potentially advantageous for digging (i.e. high coronoid and condylar processes, relatively short angular process and diastema) were present only in octodontoids; cavioids showed opposite trends, which could represent a structural constraint for fossorial habits. Chinchilloids showed intermediate features. Genera distributed in the morphospace according to their classification into superfamilial clades. Phylogenetic signal for shape components was significant along phylogeny, while the relationship between mandibular shape and ecology was non significant when phylogenetic structure was taken into account. An early evolutionary divergence in the mandible shape among major caviomorph clades would explain the observed strong phylogenetic influence on the variation of this structure.