It has become clear that the extant vertebrates are divided into three major groups, that is, hagfishes, lampreys, and jawed vertebrates.Morphological and molecular studies, however, have resulted in conflicting views with regard m their interrelationships. To clarify the phylogenetic relationships between them, 48 orthologous protein-coding gene families were analyzed. Even as the analysis of 34 nuclear gene families supported the monophyly of cyclostomes, the analysis of 14 mitochondrial gene families suggested a closer relationship between lampreys and gnathostomes compared to hagfishes. Lampreys were sister group of gnathostomes. The results of this study sup-ported the eyclostomes. Choice of outgroup, tree-making methods, and software may affect the phylogenetic prediction, which may have caused much debate over the subject. Development of new methods for tackling such problems is still necessary.