Phylogenetic analyses based on complete nucleotide sequences of mitochondrial 12S and 16S rRNA genes support the existence of three major clades within the North American phoxinins (Teleostei: Cyprinidae): the western clade, the creek chub clade and the open posterior myodome (OPM) clade. Analyses indicated strong support for the OPM clade including, among others, Mylocheilus, Pogonichthys, Tiaroga, and Rhinichthys. These latter taxa do not exhibit an OPM and previously have not been hypothesized to be related to members of the OPM clade. Parsimony analysis resolved relationships among basal members of the OPM clade and provided an opportunity to address homology of the OPM. Two hypotheses of homology of the OPM were included in analyses, with DNA sequence data allowing investigation of homology and evolution of the OPM. The OPM is best treated as a complex character exhibiting phylogenetic and ontogenetic variation. Previous attempts to simplify coding of the character do not result in hypotheses of homology that are congruent with phylogenetic hypotheses based on DNA sequence data.