Cysticercoids of the sheep tapeworm Moniezia expansa have been grown in oribatid mites in the laboratory. Three species of mite became infected: Xenillus tegeocranus, Platynothrus peltifer and Euzetes globulus, the latter not previously recorded as a host of anoplocephalines. Cysticercoids aged 15 and 28 weeks were examined with the electron microscope. Four distinct types of cells were found in the 15-week cysticercoid. Subsequent cyst development involves a transformation from a cellular to a mainly fibrous structure. The fibres, arranged in three layers, resemble collagen fibres described elsewhere. The outer epidermis of the cyst is replaced by an amorphous, electron-dense outer coat whose nature is unknown. The inner part of the cyst becomes condensed to a myelin-like structure. The scolex develops features characteristic of the adult tapeworm.