We investigated the temperature effects on the virulence, development, reproduction, and otility of two Korean isolates of entomopathogenic nematodes, Steinernema glaseri Dongrae strain and S. longicaudum Nonsan strain. In addition, we studied the growth and virulence of their respective symbiotic bacterium, Xenorhabdus poinarii for S. glaseri and Xenorhabdus sp. for S. longicaudum, in an insect host at different temperatures. Insects infected with the nematode-bacterium complex or the symbiotic bacterium was placed at $13^{circ}C,;18^{circ}C,;24^{circ}C,;30^{circ}C,;or;35^{circ}C$ in the dark and the various parameters were monitored. Both nematode species caused mortality at all temperatures tested, with higher mortalities occurring at temperatures between $24^{circ}C;and;30^{circ}C$. However, S. longicaudum was better adapted to cold temperatures and caused higher mortality at $18^{circ}C$ than S. glaseri. Both nematode species developed to adult at all temperatures, but no progeny production occurred at $13^{circ}C;or;35^{circ}C$. For S. glaseri, nematode progeny production was best at inocula levels above 20 infective juveniles/host at $24^{circ}C;and;30^{circ}C$, but for S. longicaudum, progeny production was generally better at $24^{circ}C$. Steinernema glaseri showed the greatest motility at $30^{circ}C$, whereas S. longicaudum showed good motility at $24^{circ}C;and;30^{circ}C$. Both bacterial species grew at all tested temperatures, but Xenorhabdus sp. was more virulent at low temperatures $(13^{circ}C;and;18^{circ}C)$ than X. poinarii.