The enigmatic Semimorula liquescens E.F. Haskins, McGuin. & C.S. Berry has been isolated repeatedly from dried infructescences of Lythrum salicaria collected from Seattle and Kirkland, Washington. Detailed developmental, morphological and ultrastructural studies suggested that it represents a taxon within Mycetozoa, closely allied with Myxogastria (Myxomycetes) but with unique characteristics. Phylogeny based on two genes places it with confidence in family Echinosteliidae. This species differs from a typical Echinostelium in the way spores germinate and in the lack of a stalked sporophore, the latter being a secondary loss. Semimorula liquescens therefore might be a useful negative model to search for genes inducing stalk formation during sporulation.