The in vitro susceptibilities of 103 Mycoplasma pneumoniae isolates, 14 Mycoplasma hominis isolates, 12 Mycoplasma fermentans isolates, and 24 Ureaplasma species to ABT-773, an investigational ketolide, and seven other agents were determined. For M. pneumoniae, the ABT-773 MIC at which 90% of isolates are inhibited (MIC90; ≤0.001 μg/ml) was comparable to those of azithromycin, clarithromycin, and erythromycin and at least 128- fold lower than those of levofloxacin, gatifloxacin, moxifloxacin, and doxycycline. For M. fermentans, the ABT-773 MIC90 (≤0.008 μg/ml) was 2- to 128-fold lower than those of all other agents tested. For M. hominis, the ABT-773 MIC90 (0.031 μg/ml) was equivalent to that of moxifloxacin, 2-fold lower than those of gatifloxacin and clindamycin, and 16-fold lower than that of levofloxacin. ABT-773 was equally active against doxycycline-susceptible and doxycycline-resistant organisms. The ABT-773 MICs (0.016 μg/ml) for Ureaplasma species were the lowest of those of any drug tested. The MIC90 was 4- to 64-fold lower than those of clarithromycin, azithromycin, and erythromycin and ≥16-fold lower than those of all three fluoroquinolones. Minimal bactericidal concentrations determined for a subgroup of organisms were ≤0.063 μg/ml for M. pneumoniae and 0.25 μg/ml for M. fermentans, but they were several dilutions higher for M. hominis and Ureaplasma spp. ABT-773 has great potential for further study for the treatment of infections due to mycoplasmas and ureaplasmas.