The evolutionary relationships of the various orders of placental mammals remain an issue of uncertainty and controversy. Molecular studies of mammalian phylogeny at the DNA level that include more than just a few orders are still relatively meager. Here we report results on mammalian phylogeny deduced from the coding sequence of the single-copy nuclear gene for the interphotoreceptor retinoid binding protein (IRBP). Analysis of 13 species representing eight eutherian orders and one marsupial yielded results that falsify the hypothesis that megachiropteran bats are "flying primates," only convergently resembling microchiropteran bats. Instead, in agreement with more traditional views, as well as those from other recent molecular studies, the results strongly support a monophyletic Chiroptera (micro- and megabats grouped together). The IRBP results also offer some rare molecular support for the Glires concept, in which rodents and lagomorphs form a superordinal grouping. Also in congruence with other recent molecular evidence, IRBP sequences do not support the view of a superorder Archonta that includes Chiroptera along with Dermoptera (flying lemur), Scandentia (tree shrew), and Primates. IRBP was not however, without its shortcomings as a molecular phylogenetic system: high levels of homoplasy, evident in the marsupial outgroup, did not allow us to properly root the tree, and several of the higher level eutherian clades were only weakly supported (e.g., a Carnivora/Chiroptera clade and an Artiodactyla/Carnivora/Chiroptera clade). We suggest that these shortcomings may be diminished as the phylogenetic density of the data set is increased.