For the epidemiological survey of porcine reproductive and respiratory syndrome virus (PRRSV) isolated in Japan, the open reading frame (ORF) 5 gene of 37 field isolates in Chiba prefecture from 1991 to 1999 were analyzed by restriction fragment length polymorphism (RFLP). Reverse tanscription-polymerase chain reaction (RT-PCR) amplifying the ORF 5 gene detected 35 field isolates except for 2 isolates. RFLP analysis with MluI, HincII, SacII, HaeIII and MspI demonstrated that 35 field isolates were divided into 14 distinct codes and 34 isolates were distinguished from VR2332-derived modified live PRRSV vaccine, indicating the existence of genetic diversity in PRRSV field isolates in Japan. Only one strain 98-4A had an RFLP pattern identical to the vaccine strain. Nine out of the 35 field isolates (25.7%) had the RFLP code 1-3-2-1-1 and these patterns occurred throughout the examination period. This suggests that these isolates are the prevailing strain of PRRS in Chiba prefecture. RFLP with 5 enzymes demonstrated that various strains existed in the same year, and that there were different codes on the same farm according to the year. These observations indicate that there is a genetic variation among field isolates in very limited regions and some viruses possess the ability to cause their own genomic substitutions within a herd in a short time.