One hundred and fourteen Shigella sonnei strains obtained in 1991 to 2000 were tested for their susceptibilities to 12 antimicrobial agents. Nalidixic acid (NA) resistance was found in 2 of 15 strains (13.3%) in 1993, 2 of 8 strains (25%) in 1996, one of 5 strains (20%) in 1998, 7 of 21 strains (33.3%) in 1999 and 6 of 12 strains (50%) in 2000. The incidence of resistance to NA in S. sonnei strains increased significantly during this period. Among those 19 NA resistant strains, 11 strains were derived from patients with traveler's diarrhea and 8 strains were derived from patients who had not traveled abroad before the infection, namely domestic patients. PFGE analysis with Xba I revealed that all strains tested differentiated into two major clonal clusters, one cluster consisted of strains derived from patients who had traveled to India after 1993, and another cluster included strains derived from domestic patients. Mechanism of NA resistance was examined by sequencing the quinolone resistance-determining region (QRDR) of gyrA gene. Among 19 NA resistant strains tested, 11 strains presented a change at Ser-83 to Leu and 7 strains presented a change at Asp-87 to Try (5 strains) or Asn (2 strains), whereas 3 NA sensitive strains had no change in the region. These findings indicated that this mutation in gyrA plays an important role in acquisition of Nalidixic-acid resistance in clinical isolates of S. sonnei.