Abstract: It was found that a complex was formed by the reaction of gatifloxacin (GTXC) with congo red (CGR),leading to decoloration of congo red in a slightly acidic buffer medium or in a nearly neutral buffer medium.Linear relationships were obtained between the values of decrease in absorbance and concentration of GTXC in the range of (0.05-2.98)×10-5mol·L-1,when reacted in pH range of 2.4-4.2 and measured at 616 nm,with apparent molar absorptivity of 1.84×104L·mol-1·cm-1,and in the range of (0.05-2.50)×10-5mol·L-1,when reacted in nearly neutral buffer medium of pH 4.5-7.5 and measured at 508 nm,with apparent molar absorptivity of 1.83×104L·mol-1·cm-1.A photometric method for determination of GTXC based on the above findings,in either slightly acidic medium or nearly neutral medium,was proposed.Three samples of GTXC injections were analyzed by the proposed method,giving results in conformity with the labelled values.Recovery tests were made by addition of GTXC standard solution to the injection samples,and values of recovery found were in the range of 98.5%-100.4%.