Two heterorhabditid nematode strains (TUR-H1 and TUR-H2). and their bacterial symbionts isolated from soil samples taken at the campus of the Agriculture Faculty of the University of Ankara, Turkey, were identified by molecular methods and by cross-breeding with Heterorhabditis bacteriophora. The bacterial symbionts shared >99% similarity in the 16S rDNA sequence with Photorhabdus luminescens subsp. laumondii. Results of the restriction fragment length analysis of the ITS region assigned both nematode strains to the species H. bacteriophora. Cross-breeding confirmed the species designation for strain TUR-H2. Crosses of TUR-H1 with a hybrid strain of H. bacteriophora or with TUR-H2 resulted in infertile offspring. Both strains reproduced in monoxenic cultures of the symbionts P luminescens isolated from H. bacteriophora and H. megidis and the resulting dauer juveniles retained cells of the bacteria. Reproduction on the symbionts isolated from H. indica failed. Infectivity at variable soil moisture and heat tolerance of the two heterorhabditid strains was compared with a Turkish isolate of Steinernema feltiae. Significantly more nematodes invaded the insect Galleria mellonella in a sandy soil assay at 10% water content than at lower values. A higher water content significantly reduced the invasion rate. Steinernema feltiae was better adapted to a temperature of 32degreesC than the heterorhabditid strains. Hardly any nematodes of all strains survived for longer than 4 h at 36degreesC.