According to their tissue affinities and pathogenicity, the mycoplasmas may be placed into three maingroups: (1) those associated with respiratory tract infections; (2) those involved in infections of the urogenital tract; and (3) mycoplasmas showing a particular affinity for joint tissues.On the basis of this grouping, presently available data relating several Mycoplasma and Acholeplasma species with diseases of man and animals are critically reviewed. The importance of comparative studies in the field of mycoplasmal pathology is emphasized.