Phylogenetic analysis of 69 Xenorhabdus isolates demonstrated that X. stockiae predominated, with a small number of X. miraniensis (n = 3). Isolation of X. stockiae is consistent with the previous isolation from the environment in Thailand of X. stockiae strain TH01, the recA sequence of which fell within the largest phylogenetic cluster of 52 X. stockiae isolates identified here. A further 14 isolates were most closely related to X. stockiae, but showed evidence of evolutionary divergence. The Steinernema host species associated previously with X. stockiae isolated in Thailand was reported to be S. siamkayai [6], [7], and the relationship found is this study between X. stockiae and the host nematode S. websteri is a new observation. Elsewhere, S. websteri has been reported to be associated with X. nematophila [19]. X. miraniensis has been isolated previously from Australia but not from Thailand [7]. The three X. miraniensis isolates described in this study were from the same sub-district of Chaiyaphum and were associated with S. khoisanae. This nematode host has been reported previously in association with an unknown species of Xenorhabdus in South