Abstract: Reports on outbreak of extended-spectrum β-lactamases (ESβLs) by Enterobacteraiceae and especially Klebsiella pneumoniae, are few in Saudi Arabia. This study was therefore devoted to describe the outbreak which occurred by ESβL-producing K. pneumoniae. Sixteen K. pneumoniae isolates were isolated from 16 neonatal patients hospitalized from September 2007 to December 2007 in the neonatal intensive care unit during the outbreak in Al-Qatif Hospital, Eastern Province, Saudi Arabia. These isolates were sent to microbiological laboratories, College of Pharmacy, King Saud University, for investigation. Klebsiella pneumoniae strains were found to produce antibiotic resistance and produce extended spectrum beta-lactamase. Genotypic characterization of extended spectrum beta-lactamase producing K. pneumoniae showed that all isolates carried TEM-1, SHV-1 and CTX-M-15 genes. Matting out assay revealed that all third generation cephalosporins were located on transferable plasmid. An outbreak which occurred in neonatal intensive care unit was due to CTX-M-15-producing K. pneumoniae isolates either single or in multiple clones. This is the first report of blaCTX-M-15 gene in Saudi Arabia from K. pneumoniae and the first outbreak in Saudi hospitals due to CTX-M-15 producing K. pneumoniae.