To learn more about the diversity in the Caribbean basin of Xenorhabdus and Photorhabdus in relation with their nematode host and their environment, an extensive survey was conducted in this area. A total of 77 Caribbean isolates, recovered from 72 Heterorhabditis and 5 Steinernema, were studied. ln order to type faster these numerous bacterial isolates, a method previously assessed with reference strains (see Brunel et al., in this book), Restriction Fragment Length Polymorphism analysis of PCR-amplified 16S rRNA genes, was used. The ribosomal genotypic patterns were compared to those of 40 reference strains. Most of the EPN isolates had been previously identified and ecological data about the samples origin were available. The five Xenorhabdus isolates exhibited a high degree of genetic polymorphism and only one was genotypically identical to a reference strain. ln contrast, very little variability was evidenced within the 72 Photorhabdus isolates, as only four closely related genotypes, over the 12 already identified, were detected. When we compared these results with geographical and ecological data, no correlation could be drawn, excepted those directly associated to the environmental habitat of host nematodes. ln fact, a closely relatedness between bacterial genotypes and nematode species was found. This result was particularly obvious within the Photorhabdus-Heterorhabditis couple where two bacterial genotypes were exclusively associated to H. indica and two others to H. bacteriophora. Within the Xenorhabdus-Steinernema association, new symbiont genotypes corresponded to new nematode species. The availability on one hand of a fast molecular identification of bacterial symbionts, and on the other hand the characterization with satellite DNA probes of most EPNs, allowed to compare their respective taxonomie structure on a large scale. Thereby, a congruenee was evidenced which strongly supports an early co-evolution of the symbiotic partners of both couples.