In recent years the increased consumption of diclofenac, a biologically active compound that is toxic to organisms and persistent to biodegradation, has resulted in its presence in the environment. This is the first report on the biotransformation of diclofenac by a pure bacterial strain, Raoultella sp. DD4, which is able to transform 0.6 mg/L of diclofenac in 28 days. Additionally, strain DD4 is more resistant to diclofenac than other tested organisms. The estimated value for EC50 for this strain is 1.95 g/L. This is approximately five-fold higher than the value of microbial toxic concentration MTC_avg (0.416 g/L). Moreover, genotoxicity studies have indicated that diclofenac is not a mutagenic compound.