Quality of biological control products based on entomopathogenic nematodes can be severely damaged due to exposure to high temperature surpassing 40A degrees C. The study screened 36 natural populations and 18 hybrid or inbred strains of Heterorhabditis bacteriophora for their response to high temperature. Nematodes were tested with or without prior adaptation to heat at 35A degrees C for 3 h. Five strains of H. indica and one of H. megidis were also included. Molecular identification using nuclear ribosomal DNA sequences confirmed the designation to the three Heterorhabditis spp. The mean tolerated temperature ranged from 33.3A degrees C to 40.1A degrees C for non-adapted and from 34.8A degrees C to 39.2A degrees C for adapted strain populations. H. indica was the most tolerant, followed by H. bacteriophora and H. megidis. No correlation was recorded between tolerance assessed with and without adaptation to heat, implying that different genes are involved. Correlation between heat tolerance and mean annual temperature at place of origin of the strains was weak. A high variability in tolerance among strains and the relatively high heritability (hA(2)A = 0.68) for the adapted heat tolerance recorded for H. bacteriophora provide an excellent foundation for future selective breeding with the objective to enhance heat tolerance of H. bacteriophora.