In Klebsiella pneumonia, regulation of the NifF protein (involved in N-2-fixation) was not disscussed at the posttranslational level until the recent discovery in Esherichia coli. Using a new K. pneumoniae strain MF, expressing high levels of NifF protein, we report a post-translational modification of NifF, which is different from that of E.coli. K. pneumoniae MF cultivated in L-broth and induced by IPTG had two forms of NifF protein (NifF alpha and NifF beta) that behaved similarly to the two forms of E. coli 71-18 NifF proteins (NifFI and NifFII) in non-SDS-PAGE. However, they behaved differently in response to the reduction by beta-mercaptoethanol. NifF beta in the strain MF could not be reduced to NifF alpha, while NifFII from E. coli could be reduced to NifFI. Detection of NifF protein in different N-2-fixing cells of K. pneumonia showed that only NifF beta existed, suggesting that this is the active form during N-2-fixation. We conclude that post-translational control system of NifF during N-2-fixation in K.pneumoniae is possibly different from that of E. coli.