Aichi virus is a member of the Family Picornaviridae. This virus was first isolated in 1989 from a stool specimen from a patient with oyster-associated gastroenteritis in Aichi, Japan. We analyzed the function of the 5' terminal region of the genome and the leader protein in virus replication. The results indicate that both the 5' terminal region of the genome and the leader protein are involved in viral RNA replication and encapsidation.