Extended-spectrum -lactamases, plasmid-mediated AmpC -lactamases (PABLs), and plasmid-mediated metallo--lactamases confer resistance to many -lactams. In Japan, although several reports exist on the prevalence of extended-spectrum -lactamases and metallo--lactamases, the prevalence and characteristics of PABLs remain unknown. To investigate the production of PABLs, a total of 22,869 strains of 4 enterobacterial species, Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, and Proteus mirabilis, were collected during six 6-month periods from 17 clinical laboratories in the Kinki region of Japan. PABLs were detected in 29 (0.13%) of 22,869 isolates by the 3-dimensional test, PCR analysis, and DNA sequencing analysis. PABL-positive isolates were detected among isolates from 13 laboratories. Seventeen of 13,995 (0.12%) E. coli isolates, 8 of 5,970 (0.13%) K. pneumoniae isolates, 3 of 1,722 (0.17%) K. oxytoca isolates, and 1 of 1,182 (0.08%) P. mirabilis isolates were positive for PABLs. Of these 29 PABL-positive strains, 20 (69.0%), 6 (20.7%), 2 (6.9%), and 1 (3.4%) carried the genes for CMY-2, DHA-1, CMY-8, and MOX-1 PABLs, respectively. Pattern analysis of randomly amplified polymorphic DNA and pulsed-field gel electrophoretic analysis revealed that the prevalence of CMY-2-producing E. coli strains was not due to epidemic strains and that 3 DHA-1-producing K. pneumoniae strains were identical, suggesting their clonal relatedness. In conclusion, the DHA-1 PABLs were predominantly present in K. pneumoniae strains, but CMY-2 PABLs were predominantly present in E. coli strains. The present findings will provide significant information to assist in preventing the emergence and further spread of PABL-producing bacteria.-Lactamase production is the most important factor for -lactam resistance in Gram-negative rods (16). Plasmid-mediated -lactamases, such as extended-spectrum -lactamases (ESBLs), plasmid-mediated AmpC -lactamases (PABLs), and plasmid-mediated metallo--lactamases (MBLs), hydrolyze broad-spectrum -lactams. Detection of these plasmidmediated -lactamase-producing isolates is important for epidemiological studies and hospital infection control, because plasmid-mediated genes can spread to other organisms.The Study of Bacterial Resistance in the Kinki Region of Japan (SBRK) Antimicrobial Surveillance Program was established in 1997 to monitor the predominant pathogens and antimicrobial resistance patterns of nosocomial and communityacquired infections via a broad network of clinical laboratories differing in geographic location and size. Our previously reported survey data from the Kinki region of Japan revealed the prevalence of ESBLs and plasmid-mediated MBLs (21, 30); however, the epidemiology of PABLs remains unknown. For this reason, a laboratory-based surveillance study was conducted to determine the presence and prevalence of PABLs among members of the family Enterobacteriaceae.