Mycobacterium is a variable group of acid-fast bacillus which contains pathogenic bacteria causing tuberculosis (MTC - Mycobacterium tuberculosis complex) and leprosy (M. leprae) as well as numerous nontuberculous mycobacteria (NTM) causing diseases mostly in people with immunodeficiency, although some NTM strains are capable of causing illnesses in non-immunocompromised patients. This group includes for example Mycobacterium marinum, Mycobacterium ulcerans and Mycobacterium xenopi. These microorganisms are environmental mycobacteria, present in developing countries of Africa, but they may also be transferred to other continents. The most common symptoms of diseases caused by these species are skin lesions (hyperpigmentation, tumors, ulcers) and arthritis. Because of the rarity of their occurrence, these mycobacteria are relatively poorly known. Effective ways of treating the diseases caused by these bacilli are still under study.