Microtubles are believed to function as a cytoskeleton and to provide a mechanism for cell motility and transportation of cell contents. Microfilaments are believed to be contractile. With these functions in mind, we have attempted to define the action of microtubules and microfilaments during the amoebo- flagellate transformation and microcyst formation in the myxomycete Physarum flavicomum. Experiments were performed with colchicine and vinblastine sulphate, microtubular disorganizers, and cytochalasin B, a microfilament disorganizer. The results of these experiments are discussed. Electron microscope studies of the control swarm cells of flavicomum illustrated electron dense structures resembling pericentriolar processes which have not been previously described in myxomycetes. We have also examined myxamoebae and swarm cells grown in liquid culture medium with phase-contrast, light, and electron microscopy. Cells grown in such culture exhibit "abnormally" large vacuoles which we have described.