Background: Gatifloxacin, a broad-spectrum fluoroquinolone antibiotic, has been used for the treatment of pneumonia, acute sinusitis, acute exacerbation of chronic bronchitis, and urinary tract infections. Compared to other quinolones, gatifloxacin shows higher activity and has a low incidence of photosensitivity reaction. The purpose of this study was to evaluate the interchangeability between gatifloxacin single tablet of 200 mg and two tablets of 100 mg, by comparing the relative bioavailability of the two different formulations. Methods : This study was conducted as an open-labeled, randomized, two-way crossover design in 16 healthy male subjects. Subjects were separated into two groups (groups A and B, 8 subjects per group). Subjects in group A were administered gatifloxacin 200 mg single tablet orally and one week later gatifloxacin 100 mg two tablets orally. Subjects in group B were administered in the opposite sequence in the same manner. Serial blood samples were collected till 48 hours after drug administration. Plasma concentrations of gatifloxacin were analyzed by reversed phase HPLC. Pharmacokinetic parameters were calculated by noncompartmental methods and ANOVA was used for statistical analysis. Results: The ratio of $AUO_{last}$ of gatifloxacin 200 mg single tablet to gatifloxcin 100 mg two tablets was $1.00;{pm};0.15$ $(mean;{pm};SD)$ The ratio of $C_{MAX}$ was $0.98;{pm};0.24$ $(mean;{pm};SD)$. There were no sequence, period or treatment effects on $AUO_{last}$ and $C_{max}$ by ANOVA. The 90% confidence intervals for the ratios of geometric means based on log-transformed $AUO_{last}$ and $C_{max}$ between the two regimens were 0.93-1.06 and 0.85-1.06, respectively. Conclusions : We concluded that gatifloxacin 200 mg single tablet and gatifloxacin 100 mg two tablets show similar bioavailability, and that they can be administered interchangeably.