[en] Many questions are still unanswered about the biogeography and the origin of the wood mouse in the Mediterranean area, namely on the islands where some giant forms are described. In order to characterize the genetic structure of these insular populations and their level of morphological differentiation, 232 animals originating from 30 localities situated on several western Mediterranean islands and on the continent, either along the sea coast or more inland, were trapped. Their skulls were measured and their mtDNA purified and analysed by RFLP techniques. The wood mice living on small islands are generally bigger than their relatives living on the continent whereas the size of those living on larger islands is quite the same. This kind of insular gigantism ought to be more an adaptive response to peculiar insular environmental conditions (particularly a lower predation pressure) than a consequence of a founder effect. The mtDNA of the wood mice from north-eastern Spain belongs to the previously identified northwestern group (Michaux et al., 1996). Consequently, the Pyrenees are not a biogeographic barrier to the wood mouse. The Sicilian mice, which are genetically different from all the other west European or north-African animals, are morphologically similar to the Sardinian and Italian ones.