A craniometric investigation using univariate and principal component analysis (PCA) was performed on 91 skull traits of 472 field voles, collected on nine islands and from the Jutland peninsula in Denmark in order to reveal possible geographical differences in skull shape and size. Because of missing values in the skulls due to various damages, only 186 individuals were measured for the dorsal side of the skull, 174 individuals for the ventral side and 154 individuals for the mandible. Of these traits, from the dorsal side of the skull 28 traits were measured, from the ventral side 33 traits and from the mandible 30 traits. With few exceptions, differences in skull shape were found between samples collected from the different islands, and also between samples from islands and samples from the Jutland peninsula. It is therefore suggested that field voles have a genetic differentiation between island and island and between island and mainland at the loci determining the shape of the skull. The field voles from some islands and the eastern part of the Jutland peninsula had the largest skulls compared with field voles from other islands and other parts of the Jutland peninsula. The origin of size differences is discussed and attributed to be due to several environmental factors as geographic variation in habitat quality but also as a consequence of the island syndrome.