Abstract: Competition between entomopathogenic nematode (EPN) species is still a largely neglected topic. Previous research has shown that in the competition within one insect host, nematode Steinernema affine strongly dominates over S. kraussei and suggested a possible role of symbiotic bacteria in the competition. In present study, S. affine and S. kraussei and their symbionts were reared in different combinations on Wouts agar plates, and nematode development was observed. Resulting progeny from these combinations was harvested and body size and lipid content of infective juveniles (IJs) were assessed. S. affine was able to develop, mature and produce viable progeny on the symbiont of S. kraussei. Interestingly, there was no difference in the duration of the cycle or reproduction potential, IJ size and lipid content between S. affine reared on their own symbiont and symbiont of S. kraussei. On the other hand, S. kraussei developed and reproduced well only on its own symbiont. These experiments explained the previously observed dominance of S. affine over S. kraussei. Research with more EPN species is planned to further clarify the topic.