The nature of the smallest prokaryotes with autonomous replication, i.e. the mycoplasmas, which constitute the class Mollicutes, is presented in brief. These micro-organisms are extracellular parasites of the mucous membranes in animals. Mycoplasmas may cause infections, known as mycoplasmoses. The author describes current knowledge of the pathogenicity of mycoplasmas, especially in regard to the diverse effects on the immune system and the variability of the superficial antigens. Relatively few species of pathogenic mycoplasmas cause mycoplasmoses (both sporadic and endemic diseases) which have significant socio-economic consequences. The control and prevention of endemic mycoplasmoses can only be achieved with effective diagnostic tools and an efficient structure of epidemiological surveillance. The author recommends the encouragement of biological and genetic research on the mycoplasmas and the co-ordination of the development of diagnostic tests, including evaluation and validation in various epidemiological field situations.