The arginine deiminase gene of Mycoplasma hominis was amplified by the polymerase chain reaction, and its entire nucleotide sequence was determined. This gene consists of 1227 base pairs encoding 409 amino acids, and has 35.2% guanine plus cytosine content. Nucleotide sequence homologies of the arginine deiminase gene between M. hominis and M. arginini, and between M. hominis and M. orale were 82.1 and 80.8%, respectively, suggesting that this gene is highly conserved among arginine-utilizing Mycoplasma species.