ACH-702 is a new isothiazoloquinolone with potent in vitro and in vivo activities against important bacterial pathogens, including Staphylococcus aureus. In this study, ACH-702 was found to have promising in vitro antibacterial activity against Mycobacterium tuberculosis, with MICs of <or=1 microg/ml, comparable to that of the fluoroquinolone moxifloxacin for quinolone-susceptible isolates but superior to that for quinolone-resistant isolates. Biochemical assays involving M. tuberculosis gyrase enzymes indicated that ACH-702 had significantly improved inhibitory activity compared with fluoroquinolones.