Abstract The ultrastructure of two strains of mycoplasmas ( M. orale and M. salivarium) has been studied in tissue culture by the ultra-thin section technique. This had not been done previously for mycoplasma orale in spite of its frequent occurrence in human buccal cavity. Besides forms of large size similar in appearance to bacterial L forms, small elementary particules are described. These have already been shown by others to resemble virus particles and raise difficult problems in differential diagnosis. This resemblance is further emphasized here especially since relationship with the cell surface may be compared from a morphological point of view with that observed with myxovirus and some RNA oncogenic virus. The characteristics which will enable to distinguish between an elementary body, a microvillosity and a virus particle are described.