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Physematium spp. and related genera of radiolaria (e.g., Thalassolampe and Actissd) are characterized by a large, limpid spherical cell body varying in cytoplasmic compactness, but characteristically possessing numerous small (ca. 3 nm dia.) symbionts held in fine radiating axopodia surrounding the large central capsulum. An analysis of the cytoplasmic organization of Physematium muelleri suggests that this organism has adapted to a pelagic existence by increasing surface area to enhance prey capture while conserving biomass through the development of a large internally alveolate, spheroidal cell possessing fluid-filled spaces. The thin capsular wall is supported by a network of cytoplasmic strands emanating from the perinuclear region of the intracapsulum. The fine structural organization of the cytoplasm, composition and thickness of the central capsular wall, and the amount and kind of material deposited within the perinuclear envelope appear to be more significant taxonomic discriminating characteristics than the number or kind of siliceous spicules produced surrounding the central capsulum. The possible phylogenetic relationships among some genera related to Physematium and the functional morphology of the large, fluid-filled central capsule of close relatives are presented.