Using rhodamin-phalloidin and anti-tubulin antibodies, the cytoskeleton organization of coelomic fluid cells was studied in Arenicola marina. All cells contain both fibrillar actin and tubulin in significant amounts, but their distribution depends on the stage of cell differentiation. Juvenile cells and young amebocytes have diffuse distribution of fibrillar actin, whereas numerous longitudinal bundles of actin filaments appear in the cytoplasm of intermediate (spindle-shaped) amebocytes. Moreover, other structures of the fibrillar actin appear at this stage. These are actin rings of about 2-3 mkm in diameter, which Lie close to the cell membrane. In the case of mature (filliform or fusiform) amebocytes, almost the whole fibrillar actin is concentrated into an axial rod. The average number of actin rings in filliform amebocytes is about 18, varying from 0 up 40-50 rings per individual cell. The tubulin is randomly distributed throughout the cytoplasm of differentiating cells, whereas in mature amebocytes the whole antybody-stained tubulin was confined to the axial rod. The ultrastructural data show that the axial rod is composed of 8 nm microfilaments associated with microtubules, whereas in the actin rings only microfilaments were found. Possible functions of these unusual cytoskeleton structures are discussed.