All stages in the life cycle of Demodex aurati are found in the pilo-sebaceous component of the skin: mature adults of both sexes and ova at or above the opening of the sebaceous canal and larvae and nymphs below. No mites were found in either the anagen or catagen epithelial elongations. There appeared to be no correlation between mite life cycle and stage of hair development. Mites showed no preference for hair types nor any indication of migration in 21 days, even in heavily populated areas. Overcrowding led to distension of the hair follicle, and in a few cases, to melanocyte aggregation in the dermis around this distended area. Only adult mites (both sexes) were found to leave the hair follicle in segments of excised skin. These did not reenter hair follicles but either ascended hairs or wandered across the skin until they died. The probable sequence of life cycle events is detailed with suggestions of modes of migration and transference. Both migration and transference in Demodex aurati probably occur only in the adult stage of the life cycle. It is suggested that most topical drugs are inefficient for control of such demodicids as Demodex aurati since they fail to reach the deep-seated immature stages. Most species of the genus Demodex are intimately associated with the pilo-sebaceous component of the skin. Observations by Nutting and Rauch (1961) indicated a difference in location for the various stages of Demodex aurati in this unique habitat. A recent report by Spickett (1961) shows a differential distribution for stages in the life cycle of Demodex folliculorum in the normal (uncontrolled) pilo-sebaceous system of man. This paper records the results of a study of the distribution of the stages in the life cycle of Demodex aurati, parasite of the Golden Hamster (Mesocricetus auratus) during the cycle of hair development. MATERIALS AND METHODS Observations were made on hamster skin with hairs in all stages of development-anagen, catagen, and telogen (Dry, 1926). Both naturally occurring waves of hair growth and those initiated by plucking (Collins, 1918) were studied in serial section and as sector mounts. Plucked hairs and sector mounts were also examined to determine mite relationships to hair types. The latter were identified and classified following Dry (].926) as monotrich, auchene, and zigzag. Inbred 6to 12-generation hamsters were used in all experiments. These were prepared in the following manner: Experiment A: Hairs in the telogen stage of development were plucked from one dorsal side of one male and one female hamster. Biopsies from this side and the unplucked opposite (control) side were obtained from these animals on 1, 3, 8, 17, and 21 days postplucking. Both animals possessed high mite populations as ascertained by preliminary scrapes. Experiment B: Five dorsal areas were plucked at telogen from two males and one female hamster at intervals such that biopsies were taken on the same day from areas preplucked at 1, 3, 8, 17, and 21 days. Plucked areas were approximately 1 inch square and were so spaced that at least 1/2 inch of unplucked skin separated adjacent areas. One dorsal control area was biopsied at the same time. One of the males had a heavy parasite load, the other two animals had low infestations. Experiment C: Biopsies were taken through an advancing wave of hair growth, so that all developmental stages were obtained, from the dorsal areas of one male and one female hamster, both with heavy infestations of D. aurati. Experiment D: Segments of skin, from areas in all stages of hair development, in approximately 1-inch squares were removed from heavily infested male hamsters. Segments were spread over squares of cork moistened with physiological saline and examined for mite position and activity. These Received for publication 21 September 1962.