The social amoebas (Dictyostelia) display conditional multicellularity in a wide variety of forms. Despite widespread interest in Dictyostelium discoideum as a model system, almost no molecular data exist from the rest of the group. We have constructed the first molecular phylogeny of the Dictyostelia with parallel small subunit ribosomal RNA and α-tubulin datasets, and we found that dictyostelid taxonomy requires complete revision. A mapping of characters onto the phylogeny shows that the dominant trend in dictyostelid evolution is increased size and cell-type specialization of fruiting structures, with some complex morphologies evolving several times independently. Thus, the latter may be controlled by only a few genes, making their underlying mechanisms relatively easy to unravel. TEXTMulticellular animals and plants display an enormous variety of forms, but their underlying genetic diversity is small compared with the genetic diversity of microbes. Eukaryotic microbes include a broad range of unicellular life forms, with multiple independent inventions of multicellularity. One of the most intriguing challenges in biology is to understand the reason behind the repeated occurrence of this particular evolutionary stratagem.The social amoebas, or Dictyostelia, are a group of organisms that hover on the borderline between uni-and multicellularity. Each organism starts its life as a unicellular amoeba, but they aggregate to form a multicellular fruiting body when starved. This process has been