Definition: A genus of microaerophilic to anaerobic, nonmotile bacteria (family Mycoplasmataceae) with no cell walls. Gram-negative, they are predominantly coccoidal to coccobacillary elements, approximately 0.3 mcm in diameter, which frequently grow in short filaments; colonies are generally small, 20–30 mcm in diameter, and may have no zones of surface growth. Ureaplasma hydrolyze urea with production of ammonia, and are found in the human genitourinary tract, occasionally in the pharynx and rectum. In males, they are associated with nongonococcal urethritis and prostatitis; in females, with genitourinary tract infections and reproductive failure; in neonates, they may cause pneumonia or meningitis. The type species is Ureaplasma urealyticum.