Fluoroquinolone (FQ) antibacterial compounds are frequently detected in the aquatic environment, and photodegradation is expected to play an important role in FQ fate in some sunlit surface waters. This study investigated the direct aquatic photochemistry of three FQs: norfloxacin, ofloxacin, and enrofloxacin. The direct photolysis rate of each drug exhibited strong pH dependence when exposed to simulated sunlight. For each FQ, direct photolysis rates and total light absorbance were used to calculate quantum yields for each of three environmentally relevant protonation states: a cationic, a zwitterionic, and an anionic form. In each case, quantum yields of the species varied significantly. The quantum yield for the zwitterionic form was 2-3 times higher than that of the anionic form and over an order of magnitude higher than that of the cationic form. Antibacterial activity assays were used to determine whether the loss of parent FQ due to photolysis led to loss of activity. Norfloxacin and ofloxacin photoproducts were found to be inactive, whereas enrofloxacin photoproducts were found to retain significant activity. These results are important for aiding in predictions of the potential impacts of FQs in surface waters.