The paper deals, allowing for the newest achievements in the study of mollicute biology, with main principles and characteristics which would to be used in classification of these microorganisms. The present classification of mollicutes is based on the study of classical features of cells (morphology and ultrastructure of cells, morphology of colonies, requirements to nutrition components, biochemical and serological properties) and molecular biological properties (composition of cell proteins, properties of nucleic acids, and especially, ribosomal operons and ribosomal RNA), living conditions, ecological niches, etc. The author hopes that the work will help biologists, physicians, veterinaries, phytopathologists and other persons interested in the study of mollicutes to orient themselves in determining their systematic position.