Sapoviruses are one of the major agents of acute gastroenteritis in childhood. They form a tight genetic cluster (genus) in the Caliciviridae family that regroups both animal and human pathogenic strains. No permissive tissue culture has been developed for human sapovirus, limiting its characterization to surrogate systems. We report here on the first extensive characterization of the key enzyme of replication, the RNAdependent RNA polymerase (RdRp) pol synthesizes a double-stranded RNA or labels the template 3 terminus by terminal transferase activity. Initiation of RNA synthesis occurs de novo on heteropolymeric templates or in a primerdependent manner on polyadenylated templates. Strikingly, this mode of initiation of RNA synthesis was also described for norovirus, but not for lagovirus, suggesting structural and functional homologies in the RNAdependent RNA polymerase of human pathogenic caliciviruses. This first experimental evidence makes sapovirus 3D pol an attractive target for developing drugs to control calicivirus infection in humans.