Mycoplasma genitalium is a small parasitic bacterium which lives on the ciliated epithelial cells of the primate genital and respiratory tracts.

It has one of the smallest known genomes, and is one of the smallest bacteria. Other very small bacteria are the endosymbiont Candidatus Carsonella ruddii, and the recently discovered bacterium Nanoarchaeum. The smallest known free living bacterium is Pelagibacter ubique with 1.3 Mb.