Insect-parasitic nematodes possess many of the attributes of ideal biological control agents, but intolerance to extreme temperatures can restrict their use. We examined whether heat-shock treatments could improve nematode survival and infectivity at temperatures that normally inhibit their activity (35 and 40 degrees C). Nematodes exposed to a sub-lethal temperature (35 degrees C) for 3 h with a latency period of 1-2 h at 25 degrees C killed insects at 35 and 40 degrees C. Correlative evidence was obtained between increased thermotolerance and the synthesis of 70-kDa heat-shock proteins (hsps). These results provide the first evidence of hsp synthesis in the development of thermotolerance and biological activity in the non-feeding, developmentally arrested, infective juvenile nematodes.