Transcription of the uvrD gene of Escherichia coli was studied using the Mud(Aprlac) gene fusion technique of Casadaban and Cohen [Proc. Natl. Acad. Sci. USA 76 (1979) 4530-4533]. Strains were isolated with Mud(Aprlac) inserted in both orientations and chromosome mobilisation experiments showed that transcription of uvrD was from ilvD towards metE. Constitutive expression of uvrD was approximately equivalent to 3000 protein molecules per cell. This level increased 1.5-fold following treatment with DNA damaging agents, an increase which was regulated by the recA and lexA genes. In addition, the constitutive expression of uvrD was reduced in strains containing either the recA56 mutation or a multi-copy plasmid carrying lexA+. These results indicate that uvrD is an SOS-inducible gene.