This study describes some important bio-ecological properties of Steinernema weiseri, a recently isolated entomopathogenic nematode species in Turkey. The effects of temperature on the infectivity and development of S. weiseri were determined at 5, 8, 10, 15, 20, 25 and 30 degrees C. The greatest and the fastest mortality of Galleria mellonella larvae was observed at 20 and 25 degrees C (ca 2 days) and the slowest at 5 degrees C in 11.5 days. The emergence time of the new generation of infective juveniles (U) from the host cadaver was shortest at 20 and 25 degrees C (ca 9 days) and the longest at 8 degrees C (ca 40 days). No progeny were observed at 5 and 30 degrees C. At the tested temperatures, the lowest number of new generation IJ was obtained at 8 and 10 degrees C and the highest at 15 and 20 degrees C. The life cycle, determined at 23-24 degrees C under laboratory conditions using G. mellonella as a host, took 9 days to complete. Our study suggests that the Turkish isolate of S. weiseri is adapted to cold temperatures. Steinernema weiseri has a wide host range and infects and develops well in larvae of Agrotis sp., Ceratitis capitata, Cydia splendana, Synanthedon vespiformis and Hoplocampa flava, but it infects and develops poorly in Polyphylla fullo. Larval stages of C. cossus were divided into four groups based on the diameter of their head capsules. Steinernema weiseri developed well in larval stages in Group I (the smallest head capsule) and Group 2 of Cossus cossus but, interestingly, very low infection and nematode development occurred in the Group 3 and Group 4 larval stage of C. cossus. No nematode infection was observed in Gryllotalpa gryllotalpa either.