The comparison of the genomes of two very closely related human mucosal pathogens, Mycoplasma genitalium and Mycoplasma pneumoniae, has helped define the essential functions of a self-replicating minimal cell, as well as what constitutes a mycoplasma. Here we report the complete sequence of a more distant phylogenetic relative of those bacteria, Ureaplasma urealyticum (parvum biovar), which is also a mucosal pathogen of humans. It is the third mycoplasma to be sequenced, and has the smallest sequenced prokaryotic genome except for M. genitalium. Although the U. urealyticum genome is similar to the two sequenced mycoplasma genomes, features make this organism unique among mycoplasmas and all bacteria. Almost all ATP synthesis is the result of urea hydrolysis, which generates an energy-producing electrochemical gradient. Some highly conserved eubacterial enzymes appear not to be encoded by U. urealyticum, including the cell-division protein FtsZ, chaperonins GroES and GroEL, and ribonucleoside-diphosphate reductase. U. urealyticum has six closely related iron transporters, which apparently arose through gene duplication, suggesting that it has a kind of respiration system not present in other small genome bacteria The genome is only 25.5% G+C in nucleotide content, and the G+C content of individual genes may predict how essential those genes are to ureaplasma survival.