Aim: The mountain mullet, Agonostomus monticola, is a diadromous fish wide-spread throughout Middle America, occurring in rivers along the Atlantic and Pacific slopes, as well as in the West Indies. Based on the disjunct distribution of this species, it has been hypothesized that this fish may represent more than one taxon. The purpose of this study was to conduct a robust phylogeographical analysis of A. monticola across its range, using mitochondrial and nuclear markers. We aimed to investigate the potential for cryptic diversity and the time-scale of divergence in an effort to elucidate biogeographical episodes within Middle America in relation to the evolutionary history of Agonostomus monticola. Location: North and Middle America, including the West Indies. Methods: Mitochondrial and nuclear sequence data (cyt b, S7-1) from individuals throughout the range of the species were analysed phylogenetically using maximum-likelihood methodology. The oldest known fossil mullet was used as a calibration to investigate divergence times for clades within A. monticola. Results: Results indicate four distinct lineages within the known range of A. monticola that largely correspond to oceanic basins (Caribbean, Gulf of Mexico, Pacific-A, Pacific-B). Divergence time estimates indicate early to mid-Miocene divergences for all four A. monticola clades, with Oligocene to Miocene divergences of internal nodes. Main conclusions: These findings are congruent with geological hypotheses regarding movement of the Chortís block, as well as recent studies on the age of beginning emergence of the Panama Arc.