Micro- and macrospermatozoa were found in the male genital duct and female spermatheca ofScolopendra cingulata(Scolopendromorpha), whereas sperm dimorphism was not encountered inHimantarium gabrielis(Geophilomorpha). On the basis of ultrastructural analysis, the micro-and macrosperm ofS. cingulataand the single sperm type ofH. gabrielisappear functional. They are very long, filiform cells with all the morphological parts characterizing chilopodan sperm: (1) a spiral head surmounted by an acrosome containing actin, (2) a long linear tail consisting of a 9+2 axoneme surrounded by a striated cylinder and mantle. However, the dimorphic sperm differ in nuclear size, nuclear helix, chromatin condensation, and specificity for a fluorescent nuclear dye (Hoechst 33258). The fluorescent dye stained the nuclei of the microsperm ofS. cingulataand the single sperm type ofH. gabrielis, but it did not stain the nucleus of the macrosperm ofS. cingulata.On the basis of these differences, sperm dimorphism in Scolopendromorpha seems to have a functional significance. The microsperm may be active for fertilization, whereas macrosperm may have trophic or supporting activity.