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The present study describes specific identification and characterization of the biological activity of entomopathogenic nematodes which were isolated from soil samples in a recent survey of the arid ’’ Negev ’’ region of Israel. Morphological examination indicated that the nematode populations designated IS1, IS2 and IS3 represent strains of Heterorhabditis bacteriophora Poinar. Whereas IS4 is considered a strain of H. megidis Poinar, Jackson & Klein. The identification for IS-3 and IS4 isolates was confirmed by analysis of total DNA on restriction fragment length differences of repetitive sequences, while the IS2 and IS3 isolates showed banding pattern different from that of H. bacteriophora. In infectivity assays the IS4 strain of H. bacteriophora was found to be the most pathogenic of the four new population and the HP 88 strain of H. bacteriophora against the lepidopteran species Galleria mellonella, Earias insulana, Spodoptera littoralis. IS, infectivity to the lepidopteran strain was equivalent to that of the steinernematid Steinernema carpocapsae All strain. The superior infectivity of the IS4 was also demonstrated against the scarab Maladera matrida. Viability of the H. bacteriophora HP 88 infective juveniles was reduced to 20 % when exposed to 37-degrees-C for 3 h. The level of survival of all other heterorhabditids strains was 3-4.5 fold higher than the HP 88 strain. The viability of S. carpocapsae All strain was not affected by the same heat treatment. Exposure to low relative humidity conditions (3 h at 50-60 % RH) resulted in 55-85 % reduction in most of the various nematode populations. Relatively high tolerance to the desiccation conditions was recorded with the IS1 and IS3 heterorhabditid isolates.