Klebsiella pneumoniae， one of the most important nosocomial pathogens， is becoming a major problem in health care because of its resistance to multiple antibiotics， including cephalosporins of the latest generation and， more recently， even carbapenems. This is largely due to the spread of plasmid-encoded extended-spectrum β-lactamases. However， antimicrobial agents must first penetrate the outer membrane barrier in order to reach their targets， and hydrophilic and charged β-lactams presumably diffuse through the porin channels. Unfortunately， the properties of K. pneumoniae porin channels are largely unknown. In this study， we made clean deletions of K. pneumoniae porin genes ompK35 and ompK36 and examined the antibiotic susceptibilities and diffusion rates of β-lactams. The results showed that OmpK35 and OmpK36 produced larger more permeable channels than their Escherichia coli homologs OmpF and OmpC; OmpK35 especially produced a diffusion channel of remarkably high permeability toward lipophilic (benzylpenicillin) and large (cefepime) compounds. These results were also confirmed by expressing various porins in an E. coli strain lacking major porins and the major multidrug efflux pump AcrAB. Our data explain why the development of drug resistance in K. pneumoniae is so often accompanied by the mutational loss of its porins， especially OmpK35， in addition to the various plasmid-carried genes of antibiotic resistance， because even hydrolysis by β-lactamases becomes inefficient in producing high levels of resistance if the bacterium continues to allow a rapid influx of β-lactams through its wide porin channels.