In vitro activities of omadacycline, a new aminomethylcycline, were determined for Mycoplasma and Ureaplasma spp. and compared with those of azithromycin, clindamycin, moxifloxacin, tetracycline, and doxycycline. All omadacycline MICs were <2 μg/ml. MIC90s were 0.063 μg/ml for Mycoplasma hominis, 0.25 μg/ml for Mycoplasma pneumoniae, and 2 μg/ml for Ureaplasma spp. Omadacycline had the lowest MIC90 among all drugs tested against M. hominis. Omadacycline activity was not affected by macrolide, tetracycline, or fluoroquinolone resistance.