Chilopods are shown for the first time ultrastructurally to have two types of sperm: microsperm and macrosperm. Macrosperm are among the longest animal sperm, with a maximum reported length of 3 mm. Both types share a number of constant features. The spermatozoon is filiform. A pointed cylindrical acrosome less than 10 mm long in microsperm in 70 mm in macrosperm is situated on the tip of an elongate nucleus which in the macrosperm is electron lucent. The nucleus, which has a length of 25-150 mm in microsperm and 250-400mm in macrosperm, is helical virtue of a spirally arranged surface ridge and/or coiling of the main axis. A peculiar connecting piece with a distinctive form in each order involves articulation of a capitulum at the anterior end of the axoneme with the posterior end of the nucleus. This articulation is surrounded by dense material, the centriole adjunct. No proximal centriole is present nor is a microtubule-containing distal centriole (basal body) discernible though some remnants of extreme modification of this centriole are present within the capitulum. The entire (9 + 2) axoneme, with the exception of a short terminal appendage or plume, is invested by an obliquely striated sheath (striated cylinder). In the midpiece (principal piece) the axoneme is enveloped by a more or less complex mantle consisting of helically arranged components, questionably of mitochondrial origin, which are separated by longitudinal and transverse septa (both oblique) and in species with greatest modification contain vesicles, canals, and sometimes membranous bodies. In scolopendromorphs and scutigermophs but not in geophilomorphs or, apparently, lithobiomorphs, a caudal portion of the flagellum lacks the mantle though the striated cylinder persists. The morphology of the caudal appendage (plume) appears distinctive for each order. Sperm ultrastructure suggests that lithobiomorphs are more closely related to geophilomorphs, and scolopendromorphs to scutigeromorphs, than usually though and that the Pauropoda are the plesiomorph sister-group of the Chilopoda, with which their sperm share distinctive autopomorphies.