Xenorhabdus nematophilus mutants that took longer to kill insects than did the wild type were used to determine the relationship of the physicochemical properties and outer membrane composition to bacterial interaction with the antibacterial systems of Galleria mellonella larvae and to bacterial virulence. Insect serum slowed the growth of the wild-type and mutant bacteria. This was attributed to increased spheroplast formation for the mutants. Spheroplast formation was associated with an increased sensitivity to insect lysozyme and a reduction in overall bacterial cationic charge. Increasing bacterial hydrophobicity was correlated with both increased bacterial attachment to the insect's haemocytes and the accelerated removal of the bacteria from the haemolymph. Attachment of the mutants to the insect haemocytes also increased as the bacterial lipopolysaccharide content increased, the level of prophenoloxidase activation increased, and cationic charge declined. Bacterial emergence into the haemolymph occurred in parallel with haemocyte damage but neither the total lipopolysaccharide levels in the bacteria nor the rate of bacterial emergence were associated with virulence. The rate of lipopolysaccharide release into the haemolymph influenced the rate of haemocyte damage. The contribution of outer membrane proteins to lipopolysaccharide release, bacterial adhesion to haemocytes, and virulence is discussed. Virulence reflects bacterial tolerance to the host's antibacterial defences, favouring an increase in bacteria and toxic lipopolysaccharides.