When an alien species is introduced in a new area, the number of founding individuals affects the severity of the population bottleneck, hence the new population may be distinctively different, both genetically and phenotypically, from the parent population from which it is derived. In this study we investigated the variation in shape and size of the mandible among and within three populations of the invasive Pallas’s squirrel, a tree squirrel native to SE Asia and introduced in Italy, Belgium and France. Significant differences in both size and shape of the mandible were found among all population pairs, with France being the most distinct. French squirrels showed a larger and slender mandible with a broad angular process, a restricted condyle, and a backward-oriented coronoid process. The Italian and the Belgian population differ at a lesser extent, the Italian squirrels having a lower coronoid process, a broader angular apophysis, and a restricted condyle. s. Size explained 15% of the total shape variation, but the orientation of allometric trajectories did not reveal any significant difference among populations. French squirrels showed the highest fluctuating asymmetry (both size and shape) of the right versus the left mandible, the Italians the highest directional asymmetry. Results are discussed in terms of different selective pressures in the invaded areas related to functionally mastication, and possible factors affecting fluctuating and directional asymmetry. The hypothesis of the classic mandibular two-module organization of rodent mandible (alveolar region vs ascending ramus) was confirmed both before and after correcting for size.