The aim of this project was to explore the relationship between the entomopathogenic nematode Heterorhabditis downesi and its two symbionts, Photorhabdus temperata subsp. temperata and P. temperata subsp. cinerea, and the relationship (co-existence and competition) between the two symbionts, which co occur on Bull Island in Dublin Bay. There was no effect of Photorhabdus subspecies on the reproduction capacity of H. downesi in insects. Also there was no difference due to bacterial subspecies in the number of bacteria carried by H. downesi infective juveniles (IJs). The IJs carried around 200-300 bacteria on average. Infected insect cadavers were dried for up to 61 days and then rewetted. More IJs emerged from cadavers with P. t. cinerea than from cadavers with P. t. temperata. Clearly, P. t. cinerea provides an advantage to its associated nematodes under dry conditions, such as may occur at the front of a dune system, where this subspecies predominates. There was no difference between P.t. cinerea and P.t. temperata in their ability to grow at different salt concentrations, suggesting that they have similar tolerance to osmotic stress. Both symbiont subspecies grew well together when co-cultured in liquid medium, suggesting that they do not compete strongly with each other in vitro. When insects were co-infected with both subspecies, the majority of the IJs that emerged from the insects carried P.t. cinerea. This may suggest that P.t. cinerea competes better in vivo, perhaps by competing to colonise the nematodes. A small number of IJs carried both symbionts. First generation H. downesi hermaphrodites dissected from co-infected cadavers mostly carried only one symbiont or the other, but a small proportion (12%) carried both symbiont subspecies. In a choice experiment on agar plates, first generation H. downesi developing from IJ showed a preference for P.t. cinerea. Nine secondary metabolites were identified in culture filtrate of the Bull Island Photorhabdus isolates. Only one of these, isopropylstilbene, was produced by both subspecies. One (dihydro isopropylstilebene) was produced by P.t. cinerea but not by P.t. temperata. Several other molecules were produced only by P.t. temperata, including anthroquinone pigments. Insects infected with P. t. temperata bioluminesced more intensely and emitted light at a slightly different frequency to the ones infected with P. t. cinerea. In conclusion, it is suggested that the main differences between the symbionts are in relation to cadaver defence.