Abstract: AIM: To understand different histopathological and ultrastructural changes in three forms of rosacea as well as the parasitism of Demodex species in situ . METHOD: Fourteen biopsy and dissected materials of three forms of rosacea were collected and observed under light and transmission electron microscope. RESULTS: Demodex folliculorum and Demodex brevis were found parasitizing the dilated follicles and proliferative sebaceous glands. Chronic inflammatory cells of various types were present in the dermis. In acne rosacea and rinophyma rosacea, many dilated or proliferative capillaries were viewed with thinning or stratified thickenning of the basement membrane, and degranulation of the mast cells. CONCLUSION: pathological changes of rosacea of dermatitis type is due to the parasitism of Demodex species, while the histopathological changes of other types of rosacea are caused by bacterial infection together with hair follicle mite infestation, possibly being not only related to cellular and humoral immunity, but also to types III and IV allergy.