SUMMARY
Echinostelium lunatum, a new species discovered in Puerto Rico and North Carolina, is the smallest known myxomycete. It produces typical flagellate cells, 2–6-nucleate protoplasmodia, and minute stalked sporangia that contain a lunate structure at the apex of the stalk and (1-)4-8(-14) spores. There is no capillitium. No evidence of sexuality has been observed. The phylogenetic significance of the new species is discussed. There are indications that it may have evolved from a protostelid such as Cavostelium bisporum.