The spermiophores of two tick species, the kangaroo tick, Ornithodoros gurneyi and the cave tick, Ornithodoros tholozani have been examined by scanning and transmission electron microscopy. The anterior end (head) of the spermiophore is a hemisphere covered with a hexagonal network of small projections. The rest of the spermiophore is covered with longitudinal ridges, seen in sections as cellular processes whose membranes are attached only at their anterior ends by specialized 'feet'. In the cytoplasm of the sperm cell body and just beneath the cellular processes are find filaments, which form a continuous layer in O. tholozani and bundles in O. gurneyi. Fibrils tend to be situated beneath the larger cellular processes. In scanning micrographs helical constrictions have been observed in the posterior parts of some spermiophores. It is proposed that certain of the movements observed by light microscopy in living cultures of spermiophores may be explained by contraction of the cytoplasmic filaments seen in the electron microscope.