THE false-truffles (Hymenogastrales) are a group of basidomycetous fungi that produce underground truffle-like basidiocarps. They are generally believed to be independently derived from several mushroom lineages, but extensive morphological divergence often obscures recognition of these phylogenetic connections. Comparisons of mitochondrial DNA now demonstrate a surprisingly close relationship between species of false-truffles in the genus Rhizopogon (Hymenogastraceae) and the mushroom genus Suillus (Boletaceae). The striking morphological differences separating all Suillus species from Rhizopogon imply an acceleration in the rate of morphological change relative to molecular change during the evolution of these false-truffles from their mushroom ancestors. This acceleration can best be explained by rapid morphological divergence resulting from selective pressures which may have acted on a small number of developmental genes.