Pentastomids are arthropod parasites which attain maturity in the respiratory tracts of vertebrates. All pentastomids exhibit an obvious sexual dimorphism in that fully mature females are invariably much bigger than males. Unusually though, copulation, which happens only once in the lifetime of females, occurs when both sexes are approximately the same size and the uterus of the female is undeveloped. This is because sperm is stored in spermathecae which, being positioned at the junction of the oviduct and uterus, become more remote from the vagina as the uterus elongates during development. The provision of spermathecae for sperm storage allows oocytes, shed continuously from the ovary, to be fertilized from time to time. What renders this process quite remarkable, and possibly unique, is the extreme length of the patent period. In one species this can extend up to an estimated 10 years and is associated with an egg production of about 106 eggs/female/year. Spermathecal structure provides some clues as to how prolonged sperm storage and the continuous fertilization of oocytes is accomplished. The upper region of the access duct is a narrow, chitinized tube which tapers to as little as 3 μm diameter at the point of entry into the spermatheca. The chitinous (?) lining of the spermatheca is relatively impermeable and apparently functions to totally isolate sperm from external influences: stored sperm are arranged in bundles and whorls and remain totally quiescent and inactive during the storage period. The entire spermatheca is invested by muscle fibres, the contraction of which will express small numbers of sperms through the narrow access duct. These are then reactivated by secretions from the female reproductive tract: the extreme narrowness of the spermathecal duct provides the fine adjustment of the system. The narrowness of the duct also creates problems during the process of insemination, since sperm discharged directly into the female reproductive tract would be unlikely to find the spermatheca through such a structure. Accordingly, the male cirrus is much modified to directly penetrate the spermathecal duct during copulation. Sperms, stored in the male seminal vesicle, are apparently activated prior to sperm transfer, and swim along the cirrus to be guided directly into the spermathecal lumen. Some unusual variations on the normal pentastomid life-cycle are discussed.