Comparison of the lipid composition between members of the Mycoplasmatales reveals a striking diversity of lipid structures, not only between the six genera but among species within the same genus. This is in contrast to nearly all other bacterial groups in which members of the same genus possess essentially the same lipids. There are in fact more similarities between lipids of a given species of mycoplasma and a genus of bacterium than there are between lipids of a given species of mycoplasma and a genus of bacterium than there are between mycoplasma species. Mycoplasmal lipids suggest that these organisms do not represent a phylogenetically related group at all, but are probably degenerative forms of bacteria, particularly gram-positive bacteria, which have lost the ability to synthesize a cell wall.