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Scanning electron microscopy has been employed to examine events in the release and development of mitospores of the aquatic fungus, Allomyces arbuscula. Among the salient features of spore release from the mitosporangium is the digestion of the inner matrix of the exit papillum. Hydrolysis appears to begin at the outer layer of the papillum plug matrix and probably results from activation of localized hydrolytic enzymes. The plug clearly consists of at least two different component layers. Elaboration of mitospores from the mitosporangium is depicted in several micrographs. Motile spores were induced to begin development, and the sequence of surface changes associated with the encystment process was studied. Time course studies show the retraction of the flagellum, the change from elipsoidal to spherical shape, and the deposition of the cell wall. Early in encystment, small vesicles accumulate on the surface of the plasma membrane. These enlarge and fuse to form the mature cyst wall. This surface view of cell wall deposition appears to support the possible role of gamma particles in cell wall synthesis during encystment.