Although ribosomal RNA represents the majority of cellular RNA, and ribosome synthesis is closely connected to cell growth and proliferation rates, a complete understanding of the factors that influence transcription of ribosomal DNA is lacking. Here, we show that the THO complex positively affects transcription by RNA polymerase I (Pol I). We found that THO physically associates with the rDNA repeat and interacts genetically with Pol I transcription initiation factors. Pol I transcription in hpr1 or tho2 null mutants is dramatically reduced to less than 20% of the WT level. Pol I occupancy of the coding region of the rDNA in THO mutants is decreased to ∼50% of WT level. Furthermore, although the percentage of active rDNA repeats remains unaffected in the mutant cells, the overall rDNA copy number increases ∼2-fold compared with WT. Together, these data show that perturbation of THO function impairs transcription initiation and elongation by Pol I, identifying a new cellular target for the conserved THO complex.