The dictyostelids (social amoebae) are a major group of soil microbes that hover at the borderline of true multicellularity. With the completion of the Dictyostelium discoideum genome in late 2005 and the publication of the first molecular phylogeny in late 2006, Dictyostelia is gaining increasing importance as an evolutionary model system. It is a powerful and unique system for studying the evolution of fundamental eukaryotic cell biology, from cell-cell signalling to the origin and evolution of complexity and the evolution of cooperation in social amoebae. As bacterial predators, dictyostelid amoebas also play important roles in the ecology and health of soils. The first molecular phylogeny of Dictyostelia showed that the traditional taxonomy of the group is deeply flawed, as are the evolutionary theories upon which it is based. However, this is only a first glimpse of dictyostelid evolution, and many questions about the deeper trends in dictyostelid evolution cannot yet be addressed. These include the identity of the some of the deepest branches in the tree, and the position of the root. We will develop the dictyostelid phylogeny in three important ways by (i) incorporating >50 newly identified taxa, (ii) identifying additional new taxa by culture-independent sampling, and (iii) defining the root of the tree with a multigene dataset. This proposal describes a program to characterize the biodiversity, molecular and morphological evolution of the Dictyostelia, and lay the groundwork for the study of larger trends in their ecology and role in the soil. It will be the first comprehensive studies of a group of eukaryotic amoebas.