Hair-follicle mites are the only metazoan organism commonly found in the pilosebaceous components of the eyelid of man. Our study showed that Demodex folliculorum in all stages is found in the small hair follicles and the eyelash hair follicles. This species, in adult and immature forms, consumes epithelial cells, produces follicular distension and hyperplasia, and increases keratinization leading, in eyelashes, to cuffing consisting of keratin and lipid moieties. Demodex brevis (in all stages) is present in the eyelash sebaceous glands, small hair sebaceous glands, and in the lobules of the meibomian glands. Adults and immature forms consume the gland cells in all of these loci and, when infestations are heavy, can affect the formation of the superficial lipid layer of the tear film coacervate. Comparative studies of demodicids from man and other mammals suggest that keratinization, hyperplasia, distension, and melanocyte aggregation may be even more extensive if large populations of D. folliculorum build up in the follicles of the eyelid. Large populations of D. brevis may destroy the glandular cells, produce granuloma in the eyelid, and plug the ducts of the meibomian or sebaceous glands. Further studies may incriminate either or both species, in conjunction with microorganisms, as transfer agents or synergists, or both, in producing ocular disease in man. Prevention and control of these mites must await experimental studies with infested laboratory animals (such as the squirrel monkey, Saimiri sciureus). These mites are probably most vulnerable during transfer stages, when they leave their glandular or follicular habitats.