Background & Aims: The continuously self-renewing mammalian intestinal epithelium, with high cellular turnover, depends on adequate protein synthesis for its proliferative capacity. RNA polymerase III activity is related closely to cellular growth and proliferation. Here, we studied the role of Polr3b, a large RNA polymerase III subunit, in the mammalian intestinal epithelium. Methods: We derived mice with an intestinal epithelium-specific hypomorphic mutation of the Polr3b gene, using VillinCre-mediated gene ablation. Phenotypic consequences of the Polr3b mutation on the intestinal epithelium in mice were assessed using histologic and molecular methodologies, including genetic lineage tracing. Results: The Polr3b mutation severely reduced survival and growth in mice during the first postnatal week, the period when the expansion of the intestinal epithelium, and thus the requirement for protein synthesis, are highest. The neonatal intestinal epithelium of Polr3bloxP/loxP;VillinCre mice was characterized by areas with reduced proliferation, abnormal epithelial architecture, loss of Wnt signaling, and a dramatic increase in apoptotic cells in crypts. Genetic lineage tracing using Polr3bLoxP/LoxP;Rosa26-lox-stop-lox-YFP;VillinCre mice showed that in surviving mutant mice, Polr3b-deficient dying crypts were replaced progressively by Cre-escaper cells that had retained wild-type Polr3b function. In addition, enteroids cultured from Polr3bloxP/loxP;VillinCre mice showed reduced proliferative activity and increased apoptosis. Conclusions: We provide evidence for an essential role of the RNA polymerase III subunit Polr3b in orchestrating the maintenance of the intestinal crypt during early postnatal development in mice. Keywords: Polr3b, Pol III, Intestinal Epithelium, Crypts, Enteroids