Only limited information regarding the antimicrobial susceptibilities of Mycoplasma genitalium is available because of difficulties in isolating M. genitalium strains from clinical specimens. Antimicrobial susceptibilities of 15 clinical isolates, 7 ATCC strains, and an early passage of the M30 strain were examined by the broth dilution method. Azithromycin, clarithromycin, sitafloxacin, and moxifloxacin were the most active drugs against M. genitalium, and their MIC(90)s were 0.002, 0.008, 0.125, and 0.125 mg/liter, respectively.