MULTIPLE forms of RNA polymerase activity have been isolated from developing sea-urchin embryos, rat liver nuclei and calf thymus1–3. The molecular basis of this heterogeneity is at present unknown but, whatever its basis, the crucial question seems to be whether different enzymes copy different genes. We report here the isolation and purification of two different RNA polymerases from Xenopus oocytes; only one of them is localized in the nucleolus. On the basis of localization, binding to ribosomal DNA, and response to inhibitors, we conclude that the nucleolar RNA polymerase is responsible for ribosomal RNA synthesis in vivo.