Sapovirus is an important causative agent of sporadic cases as well as of outbreaks of acute gastroenteritis in humans worldwide. A total of 603 fecal specimens collected from July 2005 to June 2006 from children with acute gastroenteritis in five localities in Japan (Maizuru, Tokyo, Sapporo, Saga, and Osaka) were screened for sapovirus by RT-PCR. It was found that 17 specimens were positive for sapovirus and it represented 2.8%. Interestingly, intragenotype recombinant sapovirus GI/1 emerged with 76.4% (13 of 17) and rapidly became the leading cause of acute gastroenteritis in Japan for the first time. The lower frequency of sapovirus GI/2 and GI/4 (each of 11.8%), which were the second prevailing genotypes, was also detected. A novel nomenclature of sapovirus was proposed, in which worldwide sapovirus strains were classified into seven genogroups. Of these, novel sapovirus genogroups VI and VII demonstrated the very low homologies, only 32.8-41.6% at the amino acid level and 43.6-49.9% at the nucleotide level, to those of sapovirus genogroups I-V. Of note, two distinct clusters of sapovirus were co-circulating in porcine. Interestingly, the worldwide sapovirus strains shared the 25 nucleotide-conserved region, covering the polymerase-capsid junction which differed according to each species due to multiple nucleotide substitutions. The finding suggests that the sapovirus recombination between human and animal hardly takes place in nature. This is also the first, to our best knowledge, demonstrating the emergence of the intragenotype recombinant sapovirus with its causing diarrheal illness in Japan.