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The Mexican Neovolcanic Plateau sharply divides the vertebrate fauna of Mesoamerica where the climate of both the neotropics and temperate North America gradually blend. Only a few vertebrate groups such as the Heroine cichlids, distributed from South America to the Rio Grande in North America, are found both north and south of the Neovolcanic Plateau. To better understand the geography and temporal diversification of cichlids at this geologic boundary, we used mitochondrial DNA sequences of the cytochrome b (cyt b) gene to reconstruct the relationships of 52 of the approximately 80 species of Heroine cichlids in Mesoamerica. Our analysis suggests several cichlids in South America should be considered as part of the Mesoamerican Heroine clade because they and the cichlids north of the Isthmus of Panama are clearly supported as monophyletic with respect to all other Neotropical cichlids. We also recovered a group containing species in Paratheraps + Paraneetroplus + Vieja as the sister clade to Herichthys. Herichthys is the only cichlid clade north of the Mexican Plateau and it is monophyletic. Non-parametric rate smoothing of cichlid cyt b sequence resulted in an estimated divergence time of approximately 6 million years for Herichthys. This temporal diversification is concordant with divergence times estimated for anurans in the genus Bufo, a group that exhibits a similar geographic distribution. Our results indicate the 5-million-year-old extension of the Mexican Neovolcanic Plateau to the Gulf Coast of Mexico has strongly influenced the current transition between the vertebrate faunas of the Neotropics and Nearctic. [Copyright &y& Elsevier]