Abstract: Three strains of Xenorhabdus kozodoii and seven of X. bovienii, obtained fromdifferent species and isolates of the genus Steinernema (S. feltiae, S. vulcanicum, S. kraussei,S. apuliae and S. ichnusae), were analysed by genotypic and phenotypic assays in order toinvestigate in vitro biofilm production, antibiotics susceptibility and growth properties. Foridentification purposes we used a genotypic analysis, such as a 16S rDNA partial sequence andERIC PCR profile. Phenotypic profiles were obtained by testing, among others: growth onselective media, haemolytic activity, ß-galactosidase. Moreover, bacterial toxin was isolated andtested in vivo on insect larvae upon direct injection into the haemolymph. The results indicatethat X. bovienii isolates present a broad phenotypic spectrum, particularly for biofilm productionand cephalosporin susceptibility, suggesting that this species underwent adaptive changes,presumably following the evolutionary divergence of Steinernema species or isolates. X. kozodoiidiverges from X. bovienii for lower levels of biofilm production and for lactose utilisation.However, both species synthesized a highly active toxin, which killed larvae in a mannerresembling direct infection by EPN.