Xenorhabdus nematophila produces type 1 fimbriae on the surface of Phase I cells. Fimbriae mediate recognition and adhesion of the bacteria to its target cell. To investigate the role of fimbriae in the biology of X. nematophila, we have produced a fimbrial mutant strain by insertional inactivation of the mrxA gene, encoding the structural subunit of type 1 fimbriae. Phenotypic characterization of the mutant revealed loss of fimbriae on the cell surface. Cell surface characteristics like dye absorption, biofilm formation, red blood cell agglutination remained unaltered. The mrxA mutant was defective in swarming on soft agar, although swimming motility was not affected. Flagellar expression was suppressed in the mrxA strain under swarming conditions, but not swimming conditions. Agglutination and cytotoxicity of the mutant to larval haemocytes was also reduced. When the mutant cells were injected in the haemocoel of the fourth instar larvae of Helicoverpa armigera, an increase in the LT50 of 9–12 h was observed relative to the wild-type strain. The nematode growth was slow on the lawn of the fimbrial mutant. The mrxA negative strain was unable to colonize the nematode gut efficiently. This study demonstrates importance of type 1 fimbriae in establishment of bacteria-nematode symbiosis, a key to successful pest management program.