Phylogenetic relationships among 18 species of mainly European muroid rodents that belong to three subfamilies were estimated using complete sequences of the mitochondrial cytochrome b gene. The inferred monophyly of the subfamilies Murinae (mice and rats) and Arvicolinae (voles, lemmings, and muskrats) is in agreement with previous studies. Within the Murinae, the morphology-based division of the genus Apodemus into three subgenera is supported by these DNA sequence data. The relationships among the different genera of the Murinae were generally poorly resolved, and the relationships of Micromys and Acomys to the other murine genera remained unresolved. Within the subfamily Arvicolinae, the relations of the genera Arvicola, Clethrionomys, and Microtus remained tentative with our data. However, within the Microtus group, there is a good molecular support for the phylogenetic relationships. These findings suggest that the origin of the different murine and arvicoline lineages was rapid, indicating an adaptive radiation with fast speciation. Copyright 2000 Academic Press.