Abstract During the Plio-Pleistocene, sharp important changes in the large mammal assemblages of the Italian Peninsula are not clearly recognizable. Sometimes, a progressive and gradual renewal can be observed, either due to local evolution of pre-existing forms or by recurrent immigrations from Asia or Central Europe. The renewal phases are probably related to climatic fluctuations. An acceleration of this phenomenon may be noticed in concurrence to the major climatic crises. Under these conditions, and taking into account that the response of single taxa to climatic and environmental variations is neither uniform nor contemporary, it is very difficult, if not impossible, to fix a limit between two faunal units. In the case of the "Villafranchian"/"Galerian" faunal transition in Italy, there are at least three possible hypotheses: a) the transition can be considered to have happened more or less at the lower Pleistocene/Middle Pleistocene boundary: b) institution of the new Mammal Age, the Protogalerian, which covers the Early Pleistocene before the Great Glacial. It is characterized by the progressive appearance of "Galerian" taxa and by persistence of the arvicolids of the Microtus (Allophaiomys) subgenus: c) the transition between Villafranchian and Galerian faunas can be established conventionally e.g. at the first appearence in Italy of Megaceroides verticornis.