The phylogenetic relationships and divergence times of 25 species of melanopline grasshoppers from western North America were estimated using mitochondrial DNA sequence data from a 1300-bp fragment of cytochrome oxidase I. Although only a few of the >350 species of Melanoplus were studied, the monophyly of the genus and observed sequence divergence suggests the group has radiated within the last 3.5 million years. Much of Melanoplus diversity is concentrated in the montane species groups endemic to the various mountain systems in western North America. Although species divergences correspond to different geologic events, the recency of species origins indicates that older geotectonic events such as mountain uplift were not involved. Sequence divergences indicate that many of the species originated during the Pleistocene, but others apparently originated during the late Pliocene. Although it appears that the diversity of Melanoplus has been generated by both large scale and localized dispersal-vicariance events (i.e., regional and interisland divergences, respectively), such inferences must be made cautiously because the rate of speciation has been high and some of the phylogenetic estimates are consequently uncertain. The short internodes separating some of the species not only restrict phylogenetic resolution, but also increase the probability that the gene genealogy may not accurately reflect species relationships because of lineage sorting. Despite these limitations, the findings of this study indicate that Melanoplus has undergone a recent radiation with rates of speciation comparable to other radiations such as the African cichlids and the drosophilids of the Hawaiian islands.