Entomopathogenic nematodes (EPNs) are widely used as biological control agents against soil-dwelling insect pests. Three commercially available species are Heterorhabditis bacteriophora Poinar (Rhabditida: Heterorhabditidae) (Hb), Steinernema carpocapsae Weiser (Rhabditida: Steinernematidae) (Sc), and Steinernema feltiae Filipjev (Sf). This study evaluated the effects of high temperature exposure on both survival and infectivity of these EPNs. Survival was assessed after diluting Hb, Sc, and Sf in water and treating for 1 to 10 h in glass vials in incubators at 30 to 45°C. Overall, Sc was the most heat tolerant, then Hb, then Sf. Treatments at 30°C had no impact on survival of any species, there was varying survival at 35 and 40°C, and treatments of 1 or 2 h at 45°C resulted in complete mortality. Product formulation was not found to have a consistent, significant effect on survival. Both Sc and Sf were treated at 30, 35, and 40°C, then two surviving infective juveniles were applied to individual Galleria mellonella Linn. (Lepidoptera: Pyralidae) larvae in well plates filled with moist sand. Larval mortality was assessed after 3 days. Doses of 5000 infective juveniles of Sc and Sf were also applied to moist growing mix in plastic cups, treated at 35 and 40°C, then 15 fungus gnat (Bradysia impatiens Johannsen; Diptera: Sciaridae; FG) larvae were added to each cup. Emerging adult FG were caught on sticky card traps and counted 2 weeks after larvae were added. Treatment at 30°C had no impact on Sc or Sf infectivity. Treatment at 35°C did not reduce infectivity of Sc, but infectivity of Sf against both hosts was reduced after 4 and 8 h exposures. After just 1 h exposure at 40°C Sf did not cause any infection, while 2 h treatment slightly reduced Sc infectivity against G. mellonella, and 4 h significantly reduced Sc infectivity against FG. Without high temperature exposure Sf caused higher infection of FG than Sc did, so growers may want to rely on Sf unless soil temperatures surpass 4 h at 35°C, or if they reach 40°C for any duration, in which case Hb or Sc may be more effective.