We detected transmissible gastroenteritis virus (TGEV) antibodies in pig farms in Tochigi prefecture, although the farms had no past record of TGEV vaccination or TGE. Among the farms, Farm A showed a high antibody incidence. We could not confirm if either TGEV or porcine respiratory coronavirus (PRCV) induced the antibodies, since conventional tests failed to discriminate PRCV from TGEV. Therefore, we conducted virological and serological examinations of this farm for 4 years to establish the etiology - TGEV or PRCV. Although no TGEV was detected, PRCVs were isolated from the nasal samples of pigs. Using a commercial ELISA kit, it was found that the antibodies detected in pigs of all the raising stages and sows were raised against PRCV but not TGEV. The phylogenetic analysis of the nucleotide sequences of the isolates showed that they were closely related to each other, and formed a separate cluster apart from the U.S.A. and European strains. In Cesarean-derived, colostrums-deprived piglets inoculated with a PRCV isolate, no clinical signs were seen, and the viruses were mainly isolated from the nasal samples. Moreover, viral genes were detected from the nasal sample of the contact pig. The result suggested that PRCV infection was located in the nasal cavity of pigs, and horizontal transmission easily occurs. From these results, PRCVs with different origins from the exotic PRCVs might be prevalent in pig farms in Japan.