RNA polymerase (RNAP) plays a crucial role in gene expression in all organisms. It is a multiprotein complex that produces primary transcript RNA. Generally, the basal transcription apparatus in archaea is simpler than the eukaryotic RNA polymerase II counterpart. To understand the structure and function of archaeal RNAP, theTON-0309gene encoding DNA-directed RNA polymerase subunit L (ToRNAP_L) fromThermococcus onnurineusNA1 was cloned and the protein was overexpressed inEscherichia coli, purified and crystallized. The purified protein was crystallized using the hanging-drop vapour-diffusion method and the crystal diffracted to 2.10 Å resolution. The crystal belonged to the hexagonal space groupP6122, with unit-cell parametersa=b= 42.3,c= 211.2 Å. One molecule was present in the asymmetric unit, with a correspondingVMof 2.5 Å3 Da−1and a solvent content of 50.0%.