Significance DNA lesions threaten cellular life and must be repaired to maintain genome integrity. During transcription, RNA polymerases (RNAPs) actively scan DNA to find bulky lesions and trigger their repair. In growing eukaryotic cells, most transcription involves synthesis of ribosomal RNA by RNAP I (Pol I), and Pol I activity thus influences survival upon DNA damage. We determined the high-resolution electron cryomicroscopy structure of Pol I stalled by a UV-induced lesion, cyclobutane pyrimidine dimer (CPD), to unveil how the enzyme manages this important DNA damage. We found that Pol I gets stalled when the lesion reaches the bridge helix, a structural element involved in enzyme advance along DNA. We identified Pol I-specific residues around the active site that contribute to CPD-induced arrest.