ABSTRACTThere are two trophic states in the life history of the protostelidClastostelium recurvatum, an amoeboflagellate state which germinates from the spore and an obligately amoeboid state which develops from the amoeboflagellate and gives rise to the sporocarp. The amoeboflagellate has an overall morphology similar to the swarm cells of myxomycetes and the amoeboflagellates of the protostelidsProtosporangiumandCeratiomyxa.Its ultrastructure is nearly identical to that ofProtosporangium, and both share a similar cell coat and a flagellar apparatus with a reduced microtubule array-3 withCeratiomyxa.The obligate amoeba is usually uninculeate, has a prominent cell coat, and a paranuclear microtubule organizing center (MTOC) that consists of arrays of microtubules and single microtubules radiating from an electron dense core. The MTOC is very similar to that of the obligate amoeba ofProtosporangium.Based on the ultrastructure of the amoeboflagellates and obligate amoebae, it is concluded thatClastosteliumis very closely related toProtosporangium, and that these two genera form a sister group toCeratiomyxa.The group containing these three genera has several characters of the life cycle which correlate well with the ultrastructural data on the trophic cells and support them as a natural group. These three genera are the most myxomycete-like protostelids and probably represent a sister group to the myxomycetes.