RNA polymerase I (Pol I) is a highly processive enzyme that transcribes ribosomal DNA (rDNA) and regulates growth of eukaryotic cells(1-4). Crystal structures of free Pol I from the yeast Saccharomyces cerevisiae have revealed dimers of the enzyme stabilized by a 'connector' element and an expanded cleft containing the active centre in an inactive conformation(5-7). The central bridge helix was unfolded and a Pol-I-specific 'expander' element occupied the DNA-template-binding site. The structure of Pol I in its active transcribing conformation has yet to be determined, whereas structures of Pol II and Pol III have been solved with bound DNA template and RNA transcript(8-10). Here we report structures of active transcribing Pol I from yeast solved by two different cryo-electron microscopy approaches. A single-particle structure at 3.8 angstrom resolution reveals a contracted active centre cleft with bound DNA and RNA, and a narrowed pore beneath the active site that no longer holds the RNA-cleavage-stimulating domain of subunit A12.2. A structure at 29 angstrom resolution that was determined from cryo-electron tomograms of Pol I enzymes transcribing cellular rDNA confirms contraction of the cleft and reveals that incoming and exiting rDNA enclose an angle of around 150 degrees. The structures suggest a model for the regulation of transcription elongation in which contracted and expanded polymerase conformations are associated with active and inactive states, respectively.