The aim of this study is to investigate the mechanisms of compensation for the lack of major porins in Klebsiella pneumoniae under different environmental conditions. Porins form water-filled channels that allow the passive transport of small molecules in and out the bacteria, a vital process to cell survival. Because of their localization in the cell, porins must play a crucial role in the adaptation and interaction between the environment and the bacteria; and also in maintaining the integrity of the cells. It has been suggested that the loss of certain porins could significantly affect the bacterial fitness and the susceptibility to antibiotics. In K. pneumoniae there are two major porins, OmpK35 and OmpK36. It has been suggested that other porins such as OmpK37 or PhoE can compensate the loss of OmpK35/36, playing an essential role for the microorganisms in the absence of these major porins. However, the mechanisms regulating this process have been poorly investigated. In order to fully understand the role of porins in bacterial adaptation and antibiotic resistance, we have constructed simple and double mutants in all the porins of K. pneumoniae wild-type strain ATCC 13883. The level of expression of porins were measured in K. pneumoniae mutants by real time RT-PCR under different conditions (low and high osmolarity) and distinct growth phases (lag, logarithmic and stationary phase) and compared with those in the wild type strain. Our data reveal that the adaptation of K. pneumoniae to external conditions in the absence of OmpK35/36 is a complex process that includes the up-regulation and down-regulation of “secondary” porins (ompK37, ompK26, phoE and lamB) and the levels of expression of these genes vary depending on the environmental circumstances and bacterial growth phase.