In spite of the variations in cell form and cell size, some mycoplasmas show several consistent and peculiar structural features: a flask- or club-shaped cell form and a more or less defined terminal structure. Organisms with these features can be observed within the species Mycoplasma pneumoniae, M. genitalium, M. pulmonis, M. gallisepticum, M. alvi, M. sualvi and Mycoplasma sp. strain 163 K. Ultrastructural peculiarities of some flask-shaped mycoplasmas are a surface nap and a cytoskeleton. With the exception of M. alvi and M. sualvi, for which detailed investigations are lacking, the flask-shaped mycoplasmas differ from the other mycoplasmas by their gliding motility and adherence properties. All of the flask-shaped mycoplasmas ferment glucose, but there are differences in the other biochemical properties investigated and in the guanine + cytosine ratio. The question remains whether the flask shape, the adherence and the motility are associated with pathogenicity, since three mycoplasmas with these properties are established pathogens. No data are available at present on the pathogenicity of the remaining four species, but several criteria suggest that at least two of them may have an etiological role in disease.