Twenty-six nalidixic acid-resistant Shigella sonnei strains isolated from 1982 to 2001 and 56 nalidixic acid-resistant mutants induced by quinolone drugs from susceptible wild strains were analyzed by sequencing the gyrA gene. All the 22 nalidixic acid-resistant isolates from 1998 to 2001 showed identical amino acid substitution of Ser to Leu (TCG --> TTG) at codon 83 while 7 different mutation types were detected in artificially induced nalidixic acid-resistant mutants. Asp87 (GGC) type was observed most commonly among mutants induced by nalidixic acid while Ser83 (TTG) type was common among mutants induced by ciprofloxacin or norfloxacin. All the isolates collected between 1998 and 2001 showed identical or nearly identical pulsed-field gel electrophoresis pattern. These results suggest that the explosive increase of S. sonnei infection after 1998 was mainly due to the spread of restricted number of clones resistant to nalidixic acid.