A Gram-negative bacterium, Xenorhabdus nematophilus, is mutualistically associated with entomopathogenic nematode, Steinernema carpocapsae. A bacterial isolate was obtained from the nematode collected in Korea and identified as X nematophilus Kl (Park et al., 1999). This study was focused on the characterization of the bacterial isolate by 16S ribosomal RNA gene (rDNA) sequence, fatty acid composition, carbon source utility, and insecticidal activity that were compared with those of other related bacteria. The universal ribosomal primers were used to amplify 1,536 bp rDNA. The 16S rDNA sequence showed high homology (equation omitted 97%) with those of foreign X nematophilus isolates, but slightly lower homologies with those of other species in the entomopathogenic bacteria. Though there was variation in fatty acid composition between two X nematophilus isolates, they shared the major fatty acid as palmitic acid (16:0), while that of X poinarii and X bovienii was oleic acid (18:1). Carbon source utility characters of the Kl isolate differed with those of the French isolate. Two X nematophilus had high insecticidal activity against the beet armyworm, Spodoptera exigua, but the Kl isolate was 18 times as potent as the French isolate. These taxonomic comparisons suggest that the Kl isolate had unique physiological characters in fatty acid composition, carbon source utility, and insecticidal activity in the species.