Recent advances in molecular phylogenetics are continuously changing our perception of the phylogenetic relationships among the main arthropod lineages: crustaceans, hexapods, chelicerates, and myriapods. Besides the intrinsic interest in unraveling the evolution of the largest animal phylum, these studies are basic to an understanding of one of the major transitions in animal evolution-i.e., the conquest of land with all its associated structural and functional adaptations. Myriapods have been traditionally considered the closest relatives of hexapods, thus implying only one origin of terrestriality for the tracheate lineage, but this view is now challenged by molecular evidence. Sequence data available to date for centipedes and millipedes are very limited, and the taxon sampling is strongly biased. The most critical gap was the scutigeromorph centipedes, which are the sister group to all remaining Chilopoda from which they probably diverged in the Silurian if not earlier. We obtained the first complete mitochondrial sequence for a representative of this clade, the house centipede. In our phylogenetic analyses of the protein-coding genes in this mitochondrial genome, along with 16 further ones representing the other major arthropod clades plus two outgroups, the myriapods formed a clade with the chelicerates. This implies that water-to-land transition occurred at least three times (hexapods, myriapods, arachnids) during the evolution of the Arthropoda. In addition, in contrast to all previous studies, our best supported topologies favor paraphyly of the myriapods with respect to the chelicerates. This would increase to four the main events of land colonization in arthropods (once for centipedes, once for millipedes).