The localization of DNA and the distribution of the cytoskeletal proteins actin, myosin and tubulin in spermatozoa of Libinia emarginata L. were the aims of this study. DAPI, a highly reactive DNA-binding agent, revealed fluorescent staining within nucleocytoplasmic compartments around the acrosome and in the radial processes extending from the central region of the spermatozoa. The sites of DAPI-DNA staining corresponded to the position of the branched chromatin fibers revealed by electron microscopy. Antisera to myosin, actin, and tubulin revealed staining at different nucleocytoplasmic sites and in radial processes of the spermatozoa. Myosin was present at the base of each of three radial extensions, whereas actin appeared throughout the nucleocytoplasmic compartment and in the radial extensions. Actin fluorescence corresponded to the 6 nm thick filaments visualized by electron microscopy forming the core of the radial processes. Although tubulin was observed throughout the cell and within radial processes by immunofluorescence staining, intact microtubules were not revealed by electron microscopy. However, SDS PAGE comparisons between Libinia sperm extracts and dogfish brain showed small amounts of protein that comigrate with alpha and beta tubulin. These results demonstrate the existence of contractile proteins (myosin, actin) and tubulin within the DNA-containing nucleocytoplasmic compartments of Libinia sperm.