Mesorhizobium loti NZP2213.1 mutant obtained after random Tn5 mutagenesis of M. loti NZP2213 was inefficient in nitrogen fixation on Lotus corniculatus. The transposon insertion was located within an ORF with a sequence similarity to a putative glycosyl transferase from Caulobacter crescentus. The results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the mutant produced LPS of the same O-chain length but only half of the entire smooth LPS, compared to that of the parental strain. A greater diversity of the anomeric region as determined by NMR spectroscopy, reflected structural differences in the mutant repeating units represented by 6-deoxytalose, 2-OAc-6-deoxytalose, and 2-OMe-6-deoxytalose. In contrast to the completely O-acetylated 6-deoxytalose in wild-type OPS only partial O-acetylation was found in the mutant. The decrease of the LPS species with O-chains seems to be correlated with 6-deoxytalose deficiency. Microscopic examination of the nodules induced by the mutant revealed disturbances in infection thread development and premature senescence of symbiosomes. The impairment of mutant-induced symbiosomes to sustain latter stages of symbiosis could be a consequence of the decreased ratio of the hydrophobic to the hydrophilic LPSs.