The list of infections, threatening patients with impaired immunological system, especially infected with HIV, prolongs systematically. Since early eighties many authors pay attention to little known type of protozoan: Microspora. More and more often new microsporidia species are described as a cause of disease, especially in patients with AIDS. We present review of literature data concerning species known up to now as pathogenic for man: Encephalitozoon cuniculi, Encephalitozoon hellem, Nosema connori and Nosema corneum, Pleistophora sp., as well as enteropathogenic for AIDS-patients-Enterocytozoon bieneusi and Septata intestinalis.