An agar growth inhibition method for serotyping Ureaplasma urealyticum is described, and the results of applying this method to serotyping 338 strains of the organism are presented. The serotyped strains consisted of cloned isolates from male patients with primary and recurrent nongonococcal urethritis (NGU), isolates from symptomatic patients with other genitourinary tract infections and disorders, and isolates from asymptomatic carriers of U. urealyticum in the genitourinary tract (controls). Among 122 male patients with NGU, serotype 4 was associated most frequently (52%) with this disease at Camp Lejeune, N.C. Seventeen percent of the isolates were type 2. The remaining isolates consisted of types 1, 3, 6, and 8 and accounted for 6 to 9% each of the serotypes isolated from the NGU group. Types 5 and 7 were not isolated. Among 91 symptomatic patients with other genitourinary tract infections and disorders, U. urealyticum type 4 also was associated most frequently (37%) with these disorders. The remaining isolates, represented by types 1, 2, 3, 6, 7, and 8, accounted for 9 to 15% each of the types isolated from this group. Type 5 was not isolated. Among 125 symptomfree carriers of U. urealyticum in the genitourinary tract, type 8 was recovered most frequently (30%), whereas type 4 was isolated next most frequently (24%). The remaining isolates consisted of types 1, 2, 3, 5, and 6 and accounted for 2 to 15% each in this asymptomatic control group. Type 7 was not isolated. Of the present eight serotypes of U. urealyticum studied in this investigation, type 4 was associated most frequently with disease (NGU) and certain other disorders of the genitourinary tract at Camp Lejeune. A previously unknown association of U. urealyticum with frequently abacteriuric, unexplained pyuria (with or without urethral pruritus and dysuria) is reported, suggesting the existence of asymptomatic Ureaplasma urethritis.