Steinernema infective juveniles (IJs) carry cells of symbiotic bacteria in their intestine and release these bacteria upon entry into insect-haemolymph. The bacteria kill the insect, providing ideal conditions for development and reproduction of the nematodes. About three Steinernema generations can develop within one insect cadaver leading to the production of thousands of IJs. Steinernema longicaudum is the first nematode for which intraspecific male-male fighting behaviour was observed (O'Callaghan, 2006). Placing 2 males in a drop of haemolymph resulted in injurious or paralysing fighting within the hour in 20% of the drops. Not lethally injured males were less successful at siring offspring. S. longicaudum males only produce sperm after several hours with a female (Ebssa et al, 2008). Such matured males fought, paralysed and killed at a higher speed than males that had not produced sperm. Previous victory also resulted in earlier fighting and paralysis and in longer fights with new partners. Prior residency, reproductive value of or presence of a female didn’t have measurable effects on the occurrence of paralysis and death. A well-established culture of the symbiotic bacteria also enhanced fighting outcome. IJs experience a different developmental pathway than juveniles that develop straight into adults, the pathway followed is determined by environmental conditions of the parental generation. The level of aggression and the influence of relatedness depended on the developmental pathway followed: IJ-males were more aggressive than non-IJ males. Evidence of fighting avoidance mechanisms (e.g. assessment) could not be established for S. longicaudum males. Other Steinernema species, from different clades, were also studied, but none was more aggressive than S. longicaudum. Types of fighting resulting in injury and possible mechanisms leading to paralysis and death were also studied.